I should probably not assume that you know how to add an attachment to a new email. But you probably know already. In case you did not, here is how to do it. Usually the attachment that you can send from the iPhone is a photo. There are also other apps that allow you to send their document (such as Pages, Numbers, Keynote, etc).
In all these cases you can just touch the “share” icon and choose email from the resulting menu of choices. The document or photo will automatically jump into a new email message and be ready for your to enter the required fields to send it.
But there are situations where you want to do things differently. For example, you have an email thread already going where someone asks you for some additional information. You want to take a photo (or more than one photo) and send them in your reply to the thread. Or you want to send a document. The fact is, whatever you can copy, you can send in an email. If it is just text then it gets pasted. If it is not simple text, it gets attached. Here’s how to do it.
In the photos app, go to the “Camera Roll” or any other Album you have that you want to send photos from. Lets say you want to send a picture. If you touch and release that picture, it automatically blows up that image. You don’t want that. That only lets you send a new email with this picture as an attachment. What you want to do is touch and hold that picture. One second later, the “Copy” menu item shows up. Touch copy. Now go to the email from where you want to reply.
To send multiple pictures this is what you do. When you are still in the “camera roll” album, touch the Share icon on the top right. Then touch the photos you want to send. They will become “checked.” You will also notice a small “copy” button at the bottom. Now touch “Copy”. Go to your email app and paste what you copied. That’s it.
Essentially you do the same thing from the list of documents when the app starts. If the item you copied is anything other than plain text, it will become an attachment. On the other hand, if you copy something from the Messages App, it will just copy the text. When you paste it, it will paste the text at the cursor.