Well when it comes to volunteering, just be sure not to get carried away with all the stuff that is going on and to focus on what you're supposed to do, shouldn't be too hard anyways, wasn't bad for me.
As for spending, its usually a hit or miss, about half of the things I got are impulse buys but also it matters on relative availability as often things are hard to come by. I usually set a budget of about $50 with another $150 for things that are hard to come by or I've been looking for for a while i.e. rare figurines or sets.
For the first time, it's a good idea to walk around places like the stands and maybe Artist Alley, the first time should be a time to get a good feel. If anything you can look at a schedule before hand and plan on the things you want to go to. And there's also always places where you'll pick up a game of anything, MonHun, Smash Bros. and maybe they have an arcade there too? Depends on the convention.