Go to Gmail.com and log in. Once you have logged in, on the left hand side there is a button above the inbox called Compose with a plus sign next to it press it.
A tab should open at the bottom of the page allowing you to compose an email. You can open as many tabs as you want and they will appear at the bottom page as well.
From the new tab, you can add the person you want to send the email to in the "To" box, and a subject in the "Subject" box.
At the bottom of the new tab, you have several options to customize your email. You can change the font by pressing the "A" button. You can also add files by pressing the paper clip button.