First you need to figure out where you're going to ride most of the time and what those trails are like. If you're doing lift-serviced downhill then you'll likely want to look into 26" or 650B(27.5"). With a budget of $1,000 you're forced to look used if you want a full suspension. You may find a couple bikes somewhat around that price, but they will be stripped down garbage. You could hop onto a nice hardtail with that budget, but that goes back to my first point on figuring out where exactly you're going to be doing most of you're riding. Mountain biking is not cheap and only gets more expensive. Last year I went from a 26" hardtail to a full suspension 29er. COMPLETELY different bikes designed to do totally different things. Where I live now my hardtail is almost useless. I spent $3,000 on the bike plus numerous upgrades with a few more coming before spring. Probably spend close to $6,000 by the time I'm done and have it how I want.
Here is a somewhat recent pic: