Pick up weapons

the title says all. how i can do that?

Not a simple answer. It all depends on the nature of your game. Here is the general outline of what needs to happen so you know where to reasearch:

  1. Detect contact with something interesting - typically this is done through collision detection, but raycasting or simply evaluating the distance between the player and objects can be used.
  2. Find out the nature of the interesting item - often this is done through tags on your game objects. Or by attaching a script to the object containing the info.
  3. Make the object disappear. Typically the object is destoryed, but it can be disabled or moved.
  4. Set a property in the player that he now has this object.