Tourists of many nationalities require a visa to enter Nepal. The most common being the tourist visa available for a duration between 15 and 90 days. These are available at entry points on arrival, otherwise an application can be made online through the Department of Immigration http://online.nepalimmigration.gov.np/tourist-visa.As with all things travel related allow sufficient time for processing. For more information on visas and extra permits required for trekking visit their website directly at http://www.nepalimmigration.gov.np/.
If you are planning on trekking one of the popular routes, Annapurna, Everest Base Camp etc in addition to Volunteering with Mission in Church you will also need a restricted area permit. Information with regards these can be found on the Immigration website. These permits are available at major tourist centres, and it is best to contact a local trekking company either before or on your arrival to discuss your plans. With passport copies and extra photographs (two per permit) these permits can be processed over the counter.
Arranging travel insurance is also your responsibility, and will be your cover for the actives you undertake with Mission in Church. It is important to chose your insurance policy carefully and ensure it covers all of your actives, volunteer work, trekking, travel and health.