The key is to pick your "theme" and then find paper goods to go with it! You can purchase Inviting Company invites (blank cards) at several retailers online - do your homework to get the best deal! All of these companies sell the products:
I mentioned investing in an invitation software. I bought Printing Press from Mountain Cow. I only invested the basic charge of $29.99 - since I was only using it to get the printing specs.
You do not need the graphics package if you are buying cards like the ones above and you do not need to invest in their font package - there are plenty of nice, free fonts out there! Mountain Cow has come up with a lot of new software choice over the years so, you actually have plenty to choose from if you want to go a different direction! I chose them because their basic layout was easy to use! It was a lot of fun doing my own invites and I still have fun doing them for friends and family for birthday parties and other celebrations!