When you sign up, you have three options; Facebook, Google or your email-address.
Facebook: Just click on the Facebook icon and you will be signed up automatically. This means you do not have to make a new password. If you choose this option, your profile photo will be added to your profile automatically.
Google: If you have a Google account, and wish to sign up with this one, you just click on the red Google icon, and you will be logged in automatically.
Email address: If you don't want to sign up with a social media account, but want to have a separate account, simply write your email address in the empty box under the icons, and click on the arrow (see below).