It's easy. Open Google Drive, click on the + somewhere down right on your screen, and select scan.
After you made a picture of your document or photo, don't be afraid if it doesn't look like expected. You can open the menu and select to crop and click on the little palette to select how to handle the photo. Black&White, Color, Color Drawing or None. For plain documents try Black&White or color drawing. For other things, like flyers or color manuals, try color or None.
If you're done, click on the checkmark and it will be saved.
Now if you do that a lot, you can place a shortcut on your homescreen or dock, and even set a folder where it should store the scanned document into. Check the video, it's very simple.