Click the photo (or photos) you want to share. Then, click the share button. Click "Mail". Fill out the form, and click the send button (looks like a paper airplane). When your computer "Whoosh"es, it's done.
People normally like you to only send a few photos by email, unless they asked for more.
People also like a subject, so they can know what the email is about before they open it.
Image: Apple