Hi Sarah. That depends on the type of finance you’re looking at getting.
If you’re getting an unsecured personal loan (as you’d from your bank, for example), you can apply whenever you like. If you are approved, the money will be transferred to you and you’ll make your first payment about a month later.
If you are taking out a secured loan (like a PCP or HP), you generally can’t apply until after you have already signed a contract on the car, as the finance will be secured against the vehicle.