I've spent a lot of time chatting with sales associates at high-end menswear stores, and my current company is involved with Savile Row tailoring so I've had the good fortune of seeing many men go through the fitting process. It really is incredible how much better a suit looks when it fits properly. That's your number 1 priority, wherever, and whatever price range you buy it.
For your first suit, the general advice I hear is to start with a navy one. In spite of its name, it's not really as blue as you think it is--it almost looks black. The reason why it's recommended so often as a first suit choice is because it looks warmer and friendlier than a straight black suit, and thus more appropriate for more situations. You'll have many opportunities to wear it. Try on both navy and black and see if you can discern the difference.
2nd suit is usually a black one, and then from there you can branch of to browns, greys, pinstripes. Of course, there's no rule that says you have to follow this sequence, especially if you know exactly what you want.
As mentioned already, be aware of the fabric you choose. I tend to favor heavier, thicker fabrics because they usually drape and keep their shape better. I cringe whenever I see trousers that are too flimsy, and cling in all the wrong places.
Most of all though, enjoy the experience! Buying your first suit is a once-in-a-lifetime event.