The first step you must take is to log into your gmail account from your browser here: http://mail.google.com.
Once you're logged in you'll be taken to your inbox. In the top of the browser window, a little to the right of center, you'll see your email address followed by some links. The words following the email address will be clickable links where these aren't but it will look more-or-less like this:
youraddress@gmail.com 
This will take you to a page with some tabs which resemble this:
We don't use forwarding so we have it turned off and so should you unless you intend to use it (that's your call).
In the POP section, item 1. you'll see some radio buttons. It's best to click on the one labeled "Enable POP for all mail (even mail that's already been downloaded)"
Next, item 2. under POP, click in the drop-down box and decide what you want google to do with their copy of your email. Google will store up to one GB of email for you and they encourage you to freely use the storage space but there are people who contend that google intends to farm the email they've stored as raw demographic data. Knowing how people are using their email is valuable information. We neither know, nor care about that. We have our own arrangements for storing email which has value to us and we don't want to suddenly stop receiving email at some distant time in the future when our gmail finally hits the storage limit so we've selected "delete Gmail's copy" and we recommend you do the same but the choice is yours. Just be sure and click on "Save Changes" before you go any further.
Be sure to have your gmail identity and password handy, click on the link for your email client and carefully follow the instructions they give.
A word of warning here, don't use their autoconfiguration tool (mentioned at the top of this page) on a Windows system if you're already using Outlook Express for an ISP address. Unless they've fixed it since I tried it, it will "clobber" your existing email identity. In Outlook Express, click on "File", scroll down to "Identities", scroll across to and click on "Add New Identity..." then, on the google page mentioned above, scroll down past the instructions for the autoconfiguration tool and follow the manual instructions.
If you don't want to tamper with your existing email client and you have plenty of disk space, you can download another email client just to use with gmail. Thunderbird, for example, is an excellent and costless email client available from the folks at Mozilla and instructions for setting it up are on the help page at Google (listed above).
Once you've set up your email client, following googles instructions, you're good to go. You can now send and receive gmail from your onboard email client.
P.S. If you don't have an invitation for a gmail account, and want one, just drop me a line, giving me a valid email address where I can send the invitation, and I'll take care of it.
Update: You no longer need an invitation to get a gmail account and you can sign up here.