Far from a secret spot, surfing in Manhattan Beach is convenient if you are visiting Los Angeles. El Porto, which is at the northern end of Manhattan Beach attracts the most surfers. Get ready to fight the crowd on any given summer day.
El Porto is basically from 45th St to Rosecrans Ave in Manhattan Beach. The entrance to the parking lot is on 45th St. See map.
Waves tends to close out pretty fast here, and so you'll see a number of folks shortboarding. The area occasionally does have its good days, with some beautiful shape and lots of shoulder to ride.
It's usually worth doing a morning session and spending the rest of the day hanging out in the area. My suggestion would be to surf in the morning, eat at Uncle Bill's Pancake House, and chill at the pier.