Sayulita's main break is one of the better places in Mexico to take a first surf lesson. The wave reforms consistently, breaks over sand, and is usually small enough that a nervous beginner can stand up on a board within a couple of hours. Several surf schools operate on the south end of the main beach, with instructors who have been teaching here for years.
Most schools offer two-hour lessons that include board rental, a rash guard, and basic instruction in paddling, popping up, and reading the wave. Group lessons run cheaper than private sessions, and groups are usually small enough to get real individual attention. Some schools also offer multi-day packages that move from the reform break to slightly more challenging conditions as you improve.
Lunazul Surf School, on Calle Marlín near the beach, is one of the established operators and a reasonable starting point for comparing what's on offer. Boards can also be rented independently for experienced surfers who don't need instruction — the south end of the beach has several rental stalls open from early morning.