Congrats on first trip! Not sure on best time to go because we've never been in December, but I understand that early December is really slow (right after Thanksgiving) so I would try for earlier in the month.
As far as travel agent or not, I'd definitely recommend no travel agent. One of the best parts of a WDW trip is the trip planning! Any info you need is right here on the DIS and there are so many veterans that can answer any questions you have.
December is a wonderful time of year to visit, but after December 21 the crowds pick up exponentially. Not necessarily a big deal, but just be prepared.
As far as to use a TA or not, I agree with the PP, planning is so much fun for me! But if you don't have time to invest in researching and planning, then use a TA. It won't cost anything!
Otherwise, join the rest of us crazy planners and do it yourself!
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