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.
After you have downloaded the Monero software (GUI and CLI alike), your antivirus or firewall may flag the executables as malware. Some antiviruses only warn you about the possible menace, others go as far as silently removing your downloaded @wallet / @daemon. This likely happens because of the integrated miner, which is used for mining and for @block verification. Some antiviruses may erroneously consider the miner as dangerous software and act to remove it.
After you install a wallet, you need to get some Monero. There are multiple ways to acquire some coins to spend, like mining or working in exchange for Monero, but the easiest way is to use an exchange and convert your fiat money into XMR. Many exchanges, centralized and decentralized, list Monero (XMR).