Search on google (or in scratch) for a running sprite. Search things like "Mario running gif" or "zombie walking gif". Once you find what you want and download, upload it under the costume tab of your main character. It should look like this:
Next we will add this code to the animation block. The sections with the post it notes need to be customized to your character.
Finally we need to adjust the height and width of the hitbox to make sure it is about the same size as your character. Don't forget to make sure your hitbox and all costumes are centered.