How would I make a Tab Targeting System?

So my question is a bit of a odd-baller if you would call it such. I was thinking of making some sort of MMO(haha) and was thinking it'd be a cool feature to have a targeting system such as one in World of Warcraft.

I honestly have no idea where to start with such a system, there are so many different variables I'd have to look at, and that's why I'm here.

Things I'm asking help for are such as: Line of sight script; Targeting in general(of course); Combat once targeted.

I know this isn't at all the way this site is suppose to work, I just needed help and had no idea where else to go. Thanks!