A traveler must know that travelling without sufficient means is not going to be successful. So a traveler must have necessary things for making a successful trek. So, a traveler must sort out his needs. It is important to have necessary belongings such as clothing. However depending on the season you come here, your needs may vary. So it is very much important to know what things are needed in what season. And also depending on the kind of trip you are going to make, and which part of Nepal, you will have to make a gear list. Following table might be helpful for you in many ways.
Also, if in case you forget to bring something that is very much essential in your trekking, then you can buy or rent them in Kathmandu. The agency with whom you are affiliated to will also help you in having the necessary equipment.
It is also recommended to buy those kinds of equipments that are very light-weight.
Necessary equipment while travelling to Nepal:
Foot wear: comfortable trekking shoes/ boots, socks, Lightweight rubber sandals (for night time, toilet walks)
Clothing: weather differs the clothing so keeping in mind about the weather a traveler must pack his clothes but in generic waterproof jacket, trekking trousers, long sleeve shirts, normally make a causal dressing during the trek in which you are comfortable in.
Hand wear: fleece gloves, warms mittens and/or gloves.
Head wear: hat (wool or fleece hat or sun hat), scarf, Head torch and extra batteries, sunglasses.
Personal Equipment: sleeping bag, backpack large enough to carry water bottles, camera, lunch, stuff sacks for keeping your gear dry and organized, two water bottles (Nalgene wide mouth bottles are the best), sunscreen and lip salve with a high SPF, insect repellent, water purification tablets (Pristine, Biox Aqua or Aqua Mira), favorite snack food, books, iPod and cards etc., trekking poles, camera with spare batteries and memory cards, insurance certificate, a fire starter, and a knife.
Travelling: duffle bag or large backpack for your personal gear on the trek (carried by a porter), a small combination padlock to secure the bag. You will need casual clothing for air travel days and time spent in Kathmandu, toiletry bag include toilet paper, soap, towel, toothbrush, etc.
Personal first aid kit: any personal medications, Diamox (optional) helps with acclimatization. At the top of all let me remind you that people go out and buy lots of new equipment for their great trek only to struggle under the weight of too much equipment and look foolish carrying it all around. It is not possible to carry each and every stuff you want so do segment the most essential goods with light weight that will make your trek more easier and comfortable.