My first response would be to seek help from an electrician or someone qualified to perform this work.
If you are going to attempt it, you will need the following items:
- electrical box
- light switch
- light fixture
- wire ( ask you local hardware store clerk what is recommended or needed to meet code in your area.

Disconnect power at breaker to your source.
From your source,
- connect wires to electrical box. then to;
- connect wires to switch then to;
- connect wires to another box at the fixture
- connect fixture to the box.

box=====switch=====box=fixture.