Assuming your shock is adjustable which it should be you will see an adjuster of some sort at the bottom or top of the spring itself that has teeth you turn that until it sits in the groove that compresses the spring the most if it looks like this one.

It may just be threaded though in which case you just tighten it like a nut until its at its desired compression.
The right tool would be a shock adjustment tool, they are fairly universal, a good spanner wrench would probably work to, channel locks and/or a pipe wrench would work too but you
will mar up the adjustment nut.
Here is a pic that shows how you would do it with the proper tool