Need Help Please Read!

Where do you go to tune a car in Horizon, ive looked everywhere but cant seem to fin it

unfortunately you are not able to in this Forza. Maybe in the next Horizon

Go to your garage or an outpost.
Go to Custom upgrade and tune away

There’s no precise tuning in Horizon so all you can do is upgrade platform and handling components. Just a tip, always put on race brakes and if you’re upgrading to the same class that your car is already in put on sport anti-roll bars and street or sport springs, if you’re upgrading two classes above use race anti-roll bars and sport or race springs. For most cars you don’t want to upgrade it more than two, maybe 3 classes above stock.