Taking a Screenshot 

To take a screenshot you can either press command + S on your keyboard or via the window menu.

Uploading a Screenshot 

To upload  a recently taken screenshot open the Screenshot viewer from the window menu or press command + shift + S.

Select the screenshot you want from the left hand side then click upload in the top right. Once it has finished uploading you will be given a URL that you can then use to post on forums or send to a friend.

Auto Upload

You can take a screenshot, upload it and copy the url all at the same time by pressing Command + alt + S on your keyboard or by clicking the link in the window menu.