The Search box found at the top of every page should provide excellent results for most of your searches. It has a default keyword search plus five additional options, each of which has been tuned to get complete results for most search types.
For those occasions where you have a particular kind of search in mind, you can use Advanced Search. It has tools that enable you to describe precisely where and how you want to search.
Find out more about Advanced Search.
Blogposts on advanced searching (with screenshots!)
