How do I make it so a gun targets a player and then shoots?

Alright, like that title say's I was wondering how to make it so, when you click let's say while holding a gun it would target a player where you're mouse is pointed. So when they jump or move you would still be targeting them.

And then I also wan't it to shoot and then have a cool down of a few seconds

Any help would be appreciated I don't know where to start or how to do this, as of I am new at scripting and hoping to learn :)