You have a case of leaf spot on that geranium. I would trim it right back until all the spotted leaves are gone and dispose of them in a garbage bag--do not compost. Make sure you clean up and dead leaves that have fallen and pick off dead or half dead leaves.

When you are trimming it try to trim back to a leaf joint. All new growth comes from a leaf joint. Then spray it with a fungicide for leaf spot. Get some time release fertilizer and sprinkle it as per package directions in the soil around the plant and scratch it into the soil so it stays there when watering.

Pruning back a geranium is the best way to promote new growth. Remember all new growth comes from a leaf joint so cut just above it