For most attachments, the answer is to send them from the app the has the file in it. Most apps have an email option. You need to have an email account set up on the iPad to do this (not use Safari and web mail). For instance, you can send a photo directly from the Photos app.
Other types of attachments sometime require other solutions. If the first answer doesn't do it, add some detail to what you are trying to do and we may have a solution for you.