sp_WhoIsActive is a great tool for seeing what is happening at a particular moment. You can also have it capture its results to a table and schedule it which is nice. Glenn Berry has some great diagnostic queries that inspect the plan cache to show you which queries in a particular database have used the most CPU, I/O, memory, etc. They are great for finding those problem queries.
There are no special teachers of virtue, because virtue is taught by the whole community. --Plato