Need some help with 3rd person shooter

okay.. i'm a noob... just started using unity for a week.. i'm trying to make 3rd person shooter... the problem is... i'm quite confused with the script. here's the problem :

  1. how to make enemy (in this case.. monster) attack with melee...
  2. how to make ability/skill shortcut? (f1, f2 and so on)
  3. how to make enemy/AI attacking random target
  4. how to make turret shoot enemy when the enemy reach X range
  5. how to make health bar decreased randomly (example 10-30points) Sorry asking waaay to much..

First step is to look at some tutorials

3D platformer is a good place to start, it contains most of what you need for a game and shooting can be added by instantiating bullet prefabs.

Short cut keys can be assigned with Input.GetKey

AI is a whole other subject but simply following the player is covered in the tutorial

You can set attack ranges with sphere colliders

health bars are covered in the tutorial

Come back after you've finished all that or ask about a specific problem for more detailed answers

This is way too much to post as a single question. You could fill a book with the answers to this lot! I think what you're looking for is more along these lines:

How can I start learning Unity Fast?