First, you have to find an image with good proportions. The length should always be longer than the width. Bigger x smaller. Since you're a beginner, single image signatures will do. Don't focus on multiple images yet, like the Kaneki Ken signature I made. Those are rather complicated.
When it comes to the effects you add, the same rules apply as with an avatar. Just experiment until it looks pretty. You have to experiment. Keep trying new shit. That's how you improve. But before you just do whatever, you have to look at the image you've cropped out and brainstorm what would really look good. It's important to experiment, but you have to do good experiments, rights? Some images would look great with brushes, others with filters, some with gradient maps, some with all 2 or all 3, etc. You get the idea.