Step 1: The Guest List
Sit down (perhaps with the school directory) and come up with a guest list of friends whose kids are in the same age group as your own. Make sure that you include enough people to make it fun and worthwhile without being overcrowded.
Then send out an evite (we love Paperless Post's Red Delicious Apples), requesting RSVPs so that you can get an accurate guest count and know how many to prepare for. Offer clear instructions on what to bring and what to expect.