While all of this together can be stressful, you'll realize that once you ask these questions, you'll be able to figure out what you want in terms of birthday invitations in no time at all!
The first thing to think about is where you want to get the birthday invitations from. The first and most popular option for birthday invitations is to get them from a store. While you can get plenty of generic invitations from a supermarket, you might also want to check out your local stationary store. It's always more fun to send out invitations that relate to the theme of the party and you can find lots of blank invitations that will do just that at a stationary store.
If you have some more time and a creative bent, you might be interested in making the invitations yourself. If this is the case, you can find a lot in the way of ideas and materials at your nearest craft store. You can start off with a basic shape of paper and then glue extras onto it. Try not to get too elaborate as you'll still have to mail these things!
When trying to decide what to put on the invitation, remember to keep it simple. In the case of a party, you can limit what you put on it to where and when the party is, and whether there are any special instructions, like if the guests should wear a costume or be prepared for the opportunity to swim in a pool. You can also insert a small request for an RSVP at the bottom, to make sure that you can get a good head count of who will be at the party.
If you're short on ideas on what the invitations should look like, just think about the party. Are you holding it somewhere special, like a beach or a club? Try to get invitations that match the theme, whether in shape or with a picture printed on the front. If you are holding the party at a place that is used to hosting celebrations, ask them if they have any specially printed invitations.
There are plenty of options available to you when you are looking for invitations, so make sure that you have a good time when picking one out!