Generally speaking, there are two approaches to this. The first way is to use the DMVs to find out what's going on. Tim Ford has put together a good "periodic table of DMVs" that may be able to help you, but this is only a reference. You're going to have to dig into each one to figure out how to best use it. He published this at http://thesqlagentman.com/periodic-table/ if you're interested in it.

The second approach is to use software to monitor your server. The activity monitor in SSMS can tell you what's going on right now, but it doesn't handle history. There are products out there (SQL Sentry, Red Gate SQL Monitor, etc.) that will keep track of history and load, but they all cost money.

To help any further, IgorMi is right - we're going to need some information about what's you're after.