If you are going to carry your own pack you must prioritize what is really important to you on backpacking trips, and consider weight as a factor. If you are not planning on backpacking this list will still help you in your packing. Some of these items may be based on personal preference.
You will have no problem securing equipment in Kathmandu for a trek. There is an endless array of shops selling trekking gear.
Also see Dave’s Shasta Trip Supplies Guide for more ideas about what to bring especially if you will be climbing in the Himalayas.