I'm not a luthier, but I play one on the Net...

Most guitars in the modern era have adjustable truss rods, the simplest and least invasive way to tweak the action. (Martins, until recently, being an exception--they have a fixed square tube in the neck, and are a job for a trustworthy pro.) They are accessible either from the peghead--theres a little plastic plate screwed onto it which you remove--or at the other end, the heelblock, accessed from inside the body. In either case, an Allen wrench is usually the tool to adjust with. Find the correct size wrench, slacken the strings, and experiment (usually from the peghead, turning the wrench clockwise will lower the action.) Do not force it--if it doesn't want to move its probably at the end of the thread and other measures are required.

If the neck is not really warped this will often correct the problem adequately. Other (trickier) measures include: deepening the slots on the nut, shaving the bottom of the saddle (plastic/ivoroid insert in the bridge that the strings actually touch), and lowering the bridge.

Then there's heat-straightening the neck--clamping it to a strong straight piece of hardwood, and leaving it with heat on overnight--this is tricky, and doesn't seem to work permanently in my experience. Sure wouldn't have it done to the Martin again. (ouch)

One other easy thing I've done with guitars with removable necks (Normans, Fender electrics) is to remove the neck and shim the joint with a bit of thin cardboard to change the neck angle. Outrageously low-tech, but it often works and can be undone if it doesn't--unlike bridge/nut surgery.

Improving sound: 1. play it a lot. 2. make sure all braces (on the underside of the top) are glued properly and not loose or cracked.

Good luck. If its a good guitar, and the simple fixes don't work, take it to a pro rather than messing with it yourself.

Bill