Mail puts all incoming email into your Inbox; the statistic after the word Inbox lets you know how many messages you haven’t yet read. New messages are also marked with light-blue dots in the main list.
The Mail icon in the Dock also shows you how many new messages you have waiting; it’s the number in the red circle. In Mail→Preferences→General, you can specify which mailbox it’s tallying up—or have it tally up all of them.
Click the Inbox folder to see a list of received messages. If it’s a long list, press Control-Page Up and Control-Page Down to scroll. (Page Up and Page Down without the Control key scroll the message pane instead.)
If you ⌘-click more than one mailbox folder, you can combine them (and their total-message counts) into one unified messages list.
Click the name of a message once to read it in the message window, or double-click a message to open it into a separate window. (If a message is already selected, pressing Return also opens its separate window.)
When you click the name of a message in the messages list (or press the and keys to highlight successive messages in that list), you see the message itself, along with any attachments, in the message pane.
Instead of reading your mail, you might prefer to have OS X read it to you, as you sit back ...