Max out your fitness skill as you'd imagine: by getting active. You can use a treadmill, punching bag, and weight lifting machine by buying one or heading to a community gym. You can also simply take jogs or swim laps in a pool. If you have Fitness Stuff, you can use the rock climbing wall. Dancing also helps with fitness. Just make sure your Sim is in a good enough mood and their needs are sufficiently met-- otherwise they may not do their workout. You could even form a club dedicated to fitness if you own the Sims 4: Get Together expansion pack.