Gmail does not use a folder system when it comes to organizing emails.  Instead it uses something called labels, unlike folders an email can have more than one label assigned to it at the same time.

To create a label:

  1. Click "labels" in the toolbar:
  2. Click on "Create New:
  3. Choose what you would like to name your label and if you would like to nest the label to create a sub-label.
  4. The new filter will now appear on the left hand side of your inbox.  If you want messages with a label to stand out you can also add a label colour:

  5. Which will put a colour text box with your label name next to the email subject line as in this example.