Since the iPad does not have a general file system, emailing with an attachment works a bit differently.
Instead of opening Mail, open the app that has the file you want to attach. The Photo's app, for instance. When you are looking at a list of files, photos, whatever, you will usually see a share icon somewhere on the screen. This with either be an arrow curved up and to the right, or the same arrow coming out of a box.
You'll either select a file, and then this arrow to get an email options. Or sometimes you select the arrow and get the option to select multiple files and then select email, or some other option.
In the Photos app you'll see your grid of pictures. When you select the icon there will be three grayed out buttons to the left. Tap to select up to 5 photos, and then select the email button.
The other option is first tap on the photo and view it. When you tap on the share icon you'll get several options, including email.
Every app does it ever so slightly different, but as long as you keep an eye out for the share icon you will usually find a way to email the file. Though there are exceptions. GoodReader's email option, for instance, is found under the Manage Files section of the app.
When you choose email, the app will generate an email with the file in the body. You just fill out the to field and subject and send it on it's way.
Mail clients that are capable of receiving this type of email will display the email this way. Those that can not will still have the attachments available the normal way.