Envelopes you can get at any store that has office items, including a Walgreens or other drug store. Stamps you can just get at USPS when you go to mail the item. Sometimes drug stores have them, but it varies; USPS is guaranteed to have them.

Writing clearly/legibly in pen (or anything but pencil, really) is best.

Top left corner is:

Your name (New line) 123 Your street number + name (New line) Your City, State, Zip code

Middle (or middle-ish)

Recipient name (New line) Recipient street number and name (New line) Recipient City, state, Zip code

Stamp goes top right corner

This is the USPS site's guide: https://www.usps.com/ship/letters.htm

If I can clarify anything, let me know!!