Finish: Your paper finish will affect how you're able to print and how much ink your invitation will take. Glossy papers take a lot more ink and longer to dry. Linen papers and papers with more texture have a much higher tendency to bleed, and so they won't work with some at_home printers where you can't change your ink settings. If you're not interested in researching finishes because you're not a weird nerd (like myself) who likes to feel on paper, a matte card stock is an easy, relatively foolproof option that will more than likely work for the printer you're using.
Buying/Finding A Design: If you don't have design experience, and you're not fond of pushing images and text around on pages (aka, does the idea of designing a flyer for a Bake Sale or some other event excite you, or does it make you hide under your desk?), you're going to need to find someone to design your DIY wedding invitations for you. Fear not! Thanks to the Internet (and Etsy, bless), there are tons of places to get an invitation design. Some are even our sponsors! Here's a short list of places to check out.