When you type an address in the To:, Cc:, or Bcc: field of a message you're composing, Gmail will suggest addresses from your Contacts list. Hit Enter to choose the name at the top of the list, or scroll down to choose a different address. If Gmail doesn't suggest the address you want, just continue typing.
Gmail will first suggest the addresses you send mail to most often. To view a complete list of your most frequently mailed contacts, click Contacts on the left side any Gmail page, then click Most Contacted.