Step 1: Open 2 Paint windows.
Step 2: In the first window, go to Image » Attributes. It should look like this:
Step 3: A window should pop up. Match your settings to mine:
Step 4: Go to the second window. Choose an image of your choice and paste it into the window.
Step 5: Using the selection tool, make a 100x100 box around the part of the image you want for your icon. It should look like this:
NOTE: You can see how big you selection is in the bottom right hand corner of your screen.
Step 6: Go to Edit » Copy
Then, go back to your first window.
Step 7: Go to Edit » Paste and paste your 100x100 selection from the original image.
This is the finished product:
You are done! You can add text to it if you would like. Make sure you save the icon as a .PNG!