Clothes and equipment depends upon the time you visit Nepal and type of activity you are undertaking. Warm clothes are preferred during winter season (November-February) while light clothes for other seasons including rain gears during monsoon. However both light and warm clothes are recommended for Kathmandu valley and mountainous region. Among spare clothing, several thinner layers are better than one thick layer. Flat soled shoe is recommended for city sightseeing, and walking boots with good tread & ankle support is best whilst trekking. No transparent clothes or shorts for women are recommended.
Most of the equipments needed for your trek are available for rent or sale in Kathmandu. Waterproof, windproof jackets and pants, well broken in footwear, head cover are also recommended. Good hiking shoes, a pair of sandals, sleeping bags, down jacket, sunglasses, warm woolen hiking socks, lightweight warm gloves, drinking water bottle, water purification tablets (which is highly recommended) a Swiss army knife, torchlight, sun block cream, toiletries, one quick drying towel, first aid kit and a very good route map are other things that come very handy.