If you're coming to Rome by car and you are staying in the city centre, the best option for parking is "Parcheggio Saba Villa Borghese". You can leave your car here and reach Piazza di Spagna in just 5 minutes.

I went to Rome by car a couple of times and always leave the car here. Pricewise is really affordable, despite being in the city centre. You likely won't need a car to visit Rome, so the best option is to leave your car somewhere safe but still close to the centre.