You have at least four options: a travel professional planning individual segments or package or DIY planning individual segments or package. Personally, I would check all options, although some may not be readily available if you're looking very far into the future.
Generally, DIY segment planning is the least expensive option, but sometimes you can get lucky and find a package that is a better value.
Determine travel dates.
Determine method of travel (air, road, etc.).
Research travel costs (add about 20% to current airfare if future dates not available. For ground travel, calculate fuel at $3 per gallon, reseach hotels/motels for overnight stays and include food.
Research WDW area lodging options and prices (on and off site).
Research ticket costs for all parks planned.
Research ground transportation or rental car costs if flying.
Research food costs.
Research cost for at-home expenses (while away): pet care, house/garden care, etc.
I probably missed a few things, so be sure to add them!
There are websites for all of the major airlines, lodging chains (and many individually owned ones) and rental car companies.
Disney has an official website--more ambience than solid information IMHO--and there are a number of unofficial websites, as well, in addition to this one. Type "Walt Disney World" into a search engine, and check the results on the first three pages. In addition to this site, I use All Ears Net and MouseSavers, as well as a couple of other sites when I'm seriously planning. Check them out and see what suits your needs.
Beyond that, keep your eyes peeled for discounts for the time period of your trip!
Happy planning!
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Taja