The most important step in caring for a painted turtle's shell is cleaning. Bacteria can lead to cracks and illness. Wipe shell weekly with a part water to vinegar bath.
Buy a 55 gallon tank, make a basking spot with smooth stones, include rocks, plants, wood etc for exploring. Feed correctly and keep the tank warm and clean.
You'll want to keep your painted turtle shell clean and do so by keeping it clean with water and a mild detergent. You'll want to use a soft tissue or cloth and nothing abrasive as that will scuff the paint. You may want to consider applying a laquer but be sure to test a small area first.