An acronym is a word, usually capitalized, where each letter stands for a word. You probably already know a lot of these in English, like ASAP and VIP.
DIY means "do it yourself" and you will see it as an adjective for things you can build or make on your own. For example, "DIY toys are popular at Christmas." This means that you have to assemble the toy yourself.
RSVP means to tell someone whether you can come to an event. It comes from the French expression respondez si vous plaît. You'll see this at the bottom of invitations, usually with some contact information and posssibly a deadline, like "RSVP to Michael by October 25th."