Use specially made car shampoo, for seats and upholstery. I once used regular carpet shampoo on my old Jeep seats, and it hardened the fabric. So now, I just use Car shampoo, which comes in a can with a built in brush. Just spray on, let it set, then vacuum.
Not all stains will come out, at least not oil stains. But it will make the carpet and seats look better.
Sold in most stores like Walmart, Kmart, and of course, auto parts stores.