Just Engaged Need Help

I just got engaged last week and need any help I can get. We are looking at next September so we have plenty of time. Right now we are trying to figure out number. What is edicate with extending, "an guest", on wedding invitations. Do you do it for anyone or just if you know they are with someone. My cousins are highschool and older so do I offer it to them or not.
Also I live in Dayton, Ohio. Does anyone know affordable services...ie. Hall or caterer. Let me know and thanks for the help.