The easiest way to find something is to type one or more words in the Search box and see what shows up in the message list--you need not even press Return. By default, Mail looks for whatever you typed in the contents of all your messages. However, you can narrow down your searches in several ways.
The first thing you may notice is that as you type in the Search field, Mail displays a list of suggestions--things it guesses you might be searching for. You're free to ignore these, but if you see what you're searching for in this list, you can select it (with your mouse, or by pressing the arrow keys followed by Return) to quickly narrow the search.
For example, if you type a portion of someone's name or email address, Mail may display the full name under a "People" heading. Select that name and Mail encapsulates it in a blue bubble it calls a search token, which enables you to read it or change its attributes more easily and with fewer errors than using old-fashioned search terms such as from:name@example.com. Similarly, if you type something resembling a date (such as October 2011 or yesterday,), Mail offers to create a token that matches messages from that date.
By default, tokens match your search term in message headers or metadata, such as To, From, Subject, Date, Status (for example, unread or flagged), Attachments, or a mailbox name--whichever seems the most likely match. If you want to search in a different location, click the arrow next to the token's category and choose a different one from the pop-up menu. For example, if you select a person and the token says From:Dan Frakes, you can click From and change it to To. One of the options on this menu is always Entire Message, which expands the search to include the contents of messages, in addition to their headers and metadata.