Here is how you make a very simple and basic icon base.
The materials you need are a picture. Any picture! I'm using this one:
First take your selection tool and set it to square.
Once you've done, select the part of the picture you want to be your icon like so:
Now copy it and paste as a new image (Ctrl+V). Now time to resize it.
Set it to the size you wish.
What you have now is what people call a base.
Now you don't want to leave it as just a base. You can add borders and text and spiff it up. Get creative!