There's plenty of on-street parking but you need to know the area, best to go for a proper car park:-

Tanners lane about 50 yards from stadium. Go past the stadium. Turn right at the lights near the jaguar garage. Turn left at the roundabout opposite the Rodney. Turn almost immediately right and the car park entrance is almost immediately on your left. Its a surface one and about £2.60 for over 2 hours.

Opposite Central station. Instead of turning right at the lights past the Rodney, carry on until just before the railway bringe and there is a car park on the right. Almost next to the one above. Similar price I think.

Golden Square. Massive multi storey above the main shopping centre. Carry on past the stadium. Through the lights opposite Wickes onto the large roundabout. Go right round this almost back to where you came from, 4th exit, A57 Liverpool, called Midland way. Go along that and you'll see Golden Square coming up on your left. Look out for the entrance, it's easy to miss. Up the spiral ramp. It's free on Sundays and shops close at 5pm. Less than 10 mins to ground.