Along with all the great stuff already I'd like to suggest that you start by working on your present bike first. Work your way through servicing all the parts of your present bike and it'll be a great way to learn where you get to see how it's supposed to look and feel before you take it apart so you know what you have to do to get it back together again.
If you're in a city or larger town I'd also suggest you check out the local library. Our own here has a couple of great books on the whole thing of servicing your bike and pictures of some of the steps.
And if you don't have a way to do it get a digital camera and learn to post good quality small pictures here on the site. That way we can help you out as you run into tough spots.
For starters you'll need a kit of some of the bicycle specific tools. Park makes an all in one kit that's very nice. I think it was Jenson that had a similar kit of the bicycle specific stuff that you then add a set of allen wrenches and regular combination metric wrenches to and you're all set. Tools for doing this stuff will set you back around $100. A bike stand is also highly recomended but you can live without one as long as you have a way to lift your bike off the ground to work on it. If you're in an apartment that won't fly so you'll want a folding stand so you can put it away between sessions. And a tarp or rubber matt to catch oil and grease droppings that goes under the stand.
While you're servicing your present bike stem to stern and learning as you go you can shop around for deals on the parts and frame for your next bike.
And why another mountain bike? What about something a little different like a cyclocross bike or a 29'er or a commuter special or a single speed or whatever? Variety is the spice of life you know...