We took DS to WDW at 4 months old because it was a family trip that was planned long before he was even thought of.
My tips: don't expect to do much unless you're going with other family members or friends. We were there for 10 or 12 days (lol I can never remember how long) with 8 other family members. They watched him a little, but they all wanted to have their fun as well.
I'm not sure when you're going, but if it's hot, make sure you have bottles with water (if OK with your ped.) to keep the baby hydrated.
We brought our huge stroller with the canopy that laid down flat for napping, but it was a PITA on the Disney buses. On the monorail we could just drive the stroller on.
Learn where the baby stations are in the parks.
Gosh, it's really late and that was 4 years ago. I can't believe I can't think of more. I'll keep thinking...