I think firstly you should list down how many guests you are inviting and decide on the venue. After deciding on the venue, then the catering, cake, goodies bags and the party theme if you want to have one.
We had a small party for dear son, and only invited some close relatives of my hub's side. We had the party at my inlaws' house, catered buffet from Neogarden, cake from PineGarden, winnie the pooh theme. Because the party was a small one, thus the planning part was easier. =)