Google's Gmail application lets you do most of the same things you can with your old email application. It also has some powerful new features, and a few things work differently than what you're used to. Here are resources for learning and mastering your new email application.
Getting Started
Gmail overview
Video (3:31) Tour the Gmail interface. View new messages, perform message actions, chat with contacts online, and more.

Sending, replying, and other Gmail basics  Quick steps for how to compose messages, add attachments, reply and forward, and print messages.
Create your signature, labels, and filters  Quick steps for organizing messages with labels and setting up other elements of your environment.
Create groups and mailing lists  Create contact groups to organize your personal contacts and to use as personal mailing lists.

Advanced Tips and Tricks
Become a Gmail Ninja (PDF) Find a wide variety of tips on reading and managing your email.
Start using keyboard shortcuts Enable keyboard shortcuts in your Gmail settings. Then quickly perform almost any Gmail task without a mouse.
Become a power searcher

(Scroll down after opening the link.)
Quickly find any message using powerful search operators and shortcuts.

Explore recommended Google Labs 
Enable offline email access, email superstars, canned responses, and a host of other useful Gmail enhancements.
Managing your Inbox: Gmail Tips and Tricks

Self-paced training (short clips - 1 hour total) Go beyond the basics to learn strategies for using labels, filters, archiving, and Google search to manage your inbox.

For even more information about using Gmail, check out the Gmail Help Center.