Plenty of reasonable advice that reflects the riding styles of those offering it . . . I'll add mine.
Don't be scared of 650cc bikes - but be respectful of them. If you are the type to push everything to the limit or can't get your body to do what your mind tells it to do (target fixation, etc), you probably want to start smaller - but if you don't have the urge to run wide-open all the time, have a firm mental control of your body and the ability to quickly process information and get your body to react to it, you are probably all right with a bigger bike on your first go-round. Only you can answer those questions, and you need to be honest with yourself, otherwise you can end up in lots of trouble (and pain).
Bikes are dirt cheap right now. You can pick up any number of small dual sports for under a grand. If you don't like it, sell it. You're going to end up doing that anyway, so just get used to it now. Unless you're like me - then you just buy them and stack them up in the back yard. Word of caution though: avoid bikes that don't have clean titles. It's just another hassle to deal with.
The MSF course was mentioned. Good place to start, but remember, it's only a start. Practice the drills every now and then, it will help you stay on top of your game. When the time comes, take a course from someone like Jim Hyde. It isn't cheap, but it's a great value if you want to really learn how to handle a big bike in the dirt.
Gear: Buy it, use it. Riding gear does no good when it is hanging in the closet. It's expensive, but not nearly as expensive as skin grafts. Helmets are *really* expensive - but not as expensive as a funeral. Remember, it's not just your actions that will kill you - it is the actions of all those cell-phone using drivers that are surrounding you.
Riding a motorcycle is kinda like traveling in Mexico - it's fun, it's relaxing, and not nearly as dangerous as everyone around you would like you to believe -- IF you go in with your eyes open & take the proper precautions.