Purchase new garage door extension springs.
How to install garage door extension springs.
The easiest way to do this is to take the broken spring to a local garage door dealer or home improvement center so they can help you find an exact match.
Since extension springs are determined by how much weight they each lift and the pulley system cuts this in half you need to replace your old springs with 200 pound extension springs.
Replacing garage door extension springs is not a difficult task provided you know all the key steps.
How to replace a garage door extension spring.