If money is an issue which it was for every high school senior I knew last year, I would always think it is cheaper to plan trips on your own if you know what you are doing(with a 7 day trip, travel agents may tack on an extra $150+ just for doing the booking for you...)--I mean, I just planned my family's entire Disneyland trip (a weeklong trip) over the past few months; it is certainly possible, and you do have time on your side. Start with a guide, my personal recommendation being Birnbaums--go through the book and figure out where you want to stay, and where you might want to eat. Next, figure out exact dates and look for plane tickets (personal recommendation: jetBlue). Once you have plane tickets, make your hotel reservations, and then start the real planning--Disney World is so large that it is good to come with at least a basic idea of what you want to do; a person who wants to relax has a very different trip from a park-a-holic like me, so figure out what approach you want to take, and don't forget those great disney extras, like horseback riding, water mice, miniature golf, water parks, etc. Once you determine all you want to see and do (keep a running list), you can determine what kind of park ticket you want. Of course, if you are like me, and love the Ultimate Park Hoppers, you could take the easier path and book a Walt Disney Travel Company package, including room and Ultimate Park Hopper tickets (get the plane tickets on your own...they will cost less)--plus a few extras. It is clearly based on your preferences--we always go with packages now; they don't save you any money, mind you, but they include some great features.
One last thing, it should be FUN, not stressful, planning a Disney vacation! Good luck!
Cheers!