In that order. Which is to say that unfortunately you would be better off buying a brand new bike than pouring money into this one. The problem with those walmart (or an department store) rigs is that they're built to look like mountain bikes, without actually functioning as such. None of the components are particularly durable, nor do they function well even when they are not broken. I've spent enough time fixing friend's walmart bikes, and working in a local coop doing the same, that I should know.
If you ride with any regularity, and especially if you actualy mountain bike (this is a mountain biking forum after all) I strongly suggest heading to a local shop and buying whatever they have that suits your budget.