Accepting a payment with the GUI is very easy. Doesn't matter if you are a merchant or an user, you will have two pages available: Receive and Merchants.
A comprehensive list of terms that you often see and their explanation. If you don't know what a word means or you would like to have more info about it, just visit the Moneropedia. Some example of often searched terms are: @node, @fungibility, @view-key, @pruning.
