There are numerous treks you can try when you are in Nepal depending on the time of the year, amount of time and money you have to spend, and the amount of experience you've had. For limited time and money, the best trekking routes would be the Langtang-Helambu trek just north of Kathmandu, and parts of the Annapurna region trek north of Pokhara. If you have more time, a trek in the Everest region or the full Annapurna circuit can be rewarding. A more difficult trek is the Kanchanjunga area trek in the far-eastern Nepal. A good trekking book is recommended if you want more details on treks.
A travel/trek guide book is best for more information. Maps are available in bookstores around Kathmandu and Pokhara.
While trekking alone can be a great way to get to know the country, deciding to trek alone deserves a second thought. Safety-wise, it is generally okay to trek alone on popular trekking route. Incidents involving trekkers do occur occasionally (and probably is on the rise). But as a general advice, you should team-up. Teaming-up can also be of great help if you ever need some medical help. During the main tourist season, you will run across other trekkers who will not mind you joining them. Also, you can find posters in the main tourist areas of Kathmandu and Pokhara looking for trekking partners. An option is also to hire a trek guide or a porter to go along with you.
Trekking with an agency can be worthwhile for those who are very tight on schedule but not on money. A trekking agency can organize a trek for you for anywhere in the upwards of $25 a day depending upon the nature of your trek. The deal normally comes with food, shelter, porters and guides. You will be traveling with other similar trekkers. If you hire an agency in Kathmandu or Pokhara, you get a much better bargain than hiring one in your home country. While traveling with an agency offers you a degree of luxury that may not be available if you go independently, it also has its own limitations. You have to stick to the group schedule; you will not be able to design your own plans; the trip is fairly expensive and your interaction with the local culture will be limited.
Hiring a porter and/or a guide can add greatly to your trek experience in Nepal, especially if this is your first time, and if you are traveling on less frequented trails, thus having to carry a heavy load (tents, food etc). An independent porter costs about $2-$3 a day, and a guide costs about $4-$6. Make sure you agree upon the wage before hiring one. You can ask your hotelier or a local trekking agency in Kathmandu or Pokhara to find one for you. You can also find them in bigger settlements along your trekking route.
If you decide to hire a porter and/or a guide, remember that you are their employer and thus should take full responsibility. You must make sure they have adequate clothes and other gear necessary for the trek. It is your responsibility to rent the gear for them. It is also your responsibility to take care of their medical requirements if they fall sick during the trek. Remember that many porters hired in the lower lands of Kathmandu and Pokhara may not be aware of the problems of trekking in high altitudes.
Except the trekking areas such as the Everest, the Annapurna and the Langtang, one requires trekking permit to visit other trekking areas. Your visa is not good enough. Trekking permits are issued very easily by the Department of Immigration Office in Kathmandu and Pokhara.You need your visa, trekking fee and two colored passport-sized pictures to get your trekking permit. Remember that you require different trekking permits to different trekking areas. Note, however, that a trekking permit does not allow you to go anywhere in the country either. If mountain-peak climbing is your desire, it falls under a whole different category, and will require a different permit.
Nepal is conservative with clothes, and your reception by locals can vary greatly on the way you dress. Men should always wear a shirt (don't go around bare chested) and long pants. In view of local customs, men should try not to wear shorts, and women should avoid them altogether. For women, a skirt of mid-calf length is preferable to slacks or pants. Slacks with sarong or skirt over them, and a (at least half-sleeved) blouse or shirt are probably most appropriate.
Besides the issue of culturally sensitive dressing, it is also important for you to make sure you have appropriate clothing to meet your needs during a trek. Good shoes are of great importance. You will be walking for up to eight hours a day. They must be sturdy and comfortable. Bring along sneakers --or if you have one, a well-broken-in pair of hiking boots-- they are sufficient for most treks. For higher altitude treks where you may have to tread snow for long hours, good boots are available for rent in Kathmandu.
Also bring along a couple of pairs of warm wool, corduroy or jeans pants (for men), a warm sweater (you can also buy beautiful ones in Nepal for a bargain) and a padded jacket, a couple of T-shirts and/or shirts. Thermal underwear can be great especially between November and February. Bring plenty of woolen and cotton socks.
Anything more specialized than all this can be easily rented or bought in Nepal for a good price.
Most of what you need during a trek is available in Kathmandu, and you can buy them or rent them once you are there. Most books on trekking will list them, check one out before you embark on your trek. If you do not have a book yet and plan to get one only once you are in Nepal, there are some things you may want to bring from home. Bring ear-plugs to help you sleep in spite of barking dogs. A battery operated short-wave radio can be helpful to listen to weather reports or the news. Also bring along a pocket knife, sunscreen, bug spray, sunglasses, photographic equipment, binoculars, a compass, a good watch with possibly an altimeter, and a day pack. Others, you can buy or rent in Kathmandu for reasonable price.
Generally your hotel or lodge will let you store your luggage with them for some nominal or no fee. As long as you lock up your bags, they are normally safe.
Though in general, you are not likely to face any emergency, you can never tell. Once again, a good book on trekking will give you details on what to do in case of emergency. In cases of non-urgent situation, you may have to be carried to the nearest health-post or airfield. If the situation is more serious, send word to the nearest village with radio service for a helicopter evacuation. It costs in the neighborhood of $1200 - $2000 for a helicopter evacuation, and generally a guarantee for payment is required before the helicopter actually takes off. Registering with your embassy can greatly speed the process.
Get a good travel book to guide you on health matters. There are plenty available in Kathmandu, if you can't find one in a bookstore near you. Stephen Bezruchka's book called Trekking in Nepal addresses health issues in excellent detail. Don't forget to take a first-aid kit: the details of which are also mentioned in most trek books. All of what you will need to take along can be purchased in Kathmandu, so don't bother carrying stuff from home. Read the health guidance in Health and Insurance section and Dining and Drinking section for more details.
Besides minor ailments stomach problems, blisters, cold and headaches, the most important health problem you may run into is altitude sickness. You will not have to worry about it in Kathmandu, Pokhara or other lower places. But once you are on the trekking trails and above 3000m in altitude, watch out for its symptoms. Refer to a good travel book for details on how to recognize altitude sickness and what to do about it. As a suggestion for travel planning, you may want to plan for "rest days" at about 3,700m - 4,300m altitude levels. This means sleeping at the similar altitude for two nights. Also remember that you should not try to climb too high too soon: about 450m per day is the recommended amount.
The weather is probably the best guide for deciding when to plan your trip to Nepal. October and November are considered the best times of the year. The monsoon will have just ended, and clear skies with optimal temperature will prevail. The main festivals of Dashain and Tihar (Hindu equivalent of Christmas in terms of festivity) fall during these months. However, this is also the busiest tourist season, and the main tourist centers and trekking trails tend to be crowded with travelers like you. The tourist flow ebbs a little, but not significantly, between the winter months of December and mid-February. It catches up once again between mid-February and mid-April. From mid-June to early October, it's the monsoon, during which time it rains almost everyday and most of the Himalayas are hidden behind the clouds. Check the weather section of this FAQ for more details on weather. In short, plan to visit Nepal between October and May, keeping in mind that October-November and February-March are the best times (but crowded with other travelers).