How to add a mobileme account
This tutorial assumes you have an iPhone, and a mobileme account. Be sure you are either connected to a wi-fi network, or you've activated your wireless internet account (often referred to as the 3G network) provided by your cell phone service provider.
From the home screen choose the "Settings" button

Next select "Mail, Contacts, Calendars"

From here we want to select "Add account". (Please note: If you already have an email account setup, you will see it listed under the "Accounts" section. You can add additional email accounts as needed).

For this tutorial, we'll choose mobileMe. As you can see there are a number of email account types you can add. Each one will be similar to this process, though there may be settings that are unique to each account type.

Now we enter the settings for our mobileme account. Be sure to use the same login as when you signed up for the mobileme service. If you had an existing .mac account, the login will be the same.

Click "Save" and if you entered the account information correctly, you should see this screen where you can choose what information you want sync'd with your mobileme account

If you choose not to select anything to sync, you can always come back to these settings by choosing "Settings" > "Mail, Contacts, Calendar" and now under the "Accounts" section, you'll see your mobileme account. Clicking the account will bring up this same dialog.