Im very new to customizing my guitar. I have a Jimmie Vaughn Tex Mex Strat. Looking to put a humbucker pickup if possible. This is the first time I'm going to try to switch out the pickups. I like to play metal, please give me some recomendations on some good pickups and am I able to put in a humbucker pickup if I dont already have one? Is this hard to do? And also I always read people saying the action is high... can someone please explain to me how to lower / higher action and what does that actually do?
You would need a new pickguard, and possibly to cut a bigger cavity to fit the humbucker. However, a lot of strats have big cavities anyway, and you can fit a humbucker in there quite easily. Check before you buy the pickup.

I would recommend a Bill Lawrence L-500XL, a Duncan Distortion, or Duncan Custom. Perhaps also a DiMarzio of some sort...Evolution, anyone? They have a great lead tone.

action adjustment