This is my very first developer position and I'd like to know what I should expect and what I should be doing to prepare.
Unless you're a student finishing your degree in the spring, it's unusual for a company to offer a position 7+ months before the start date. It might help us understand your situation if you'd explain more about that.
As this is an entry-level position presumably with a large corporation, it's a good bet that the company will train you on the specific tools and processes that you'll be using. It might not hurt to ask if there's anything you can or should bone up on in preparation for starting, but trust that they know what they're doing when it comes to hiring and training new people and try not to worry about it too much. Instead, focus on what's in front of you right now. If you're still in school, concentrate on finishing this semester well and learning as much as you can in the spring.