Guam is good for surfing but I wouldn't say great. Despite the high winds, Guam has hollow reefs and the swells are somewhat inconsistent. Still, coming here for surfing is well worth it especially from July through September, and October through March. As far as beaches, Rick’s Reef (below the Sheraton Hotel) is one of the best beaches for surfing in Guam because of its easy access.
ideal for beginners Guam is probably the hardest place to learn to surf, even if you are experienced. The beaches in Guam have shallow reefs with ever-changing tides and rough current. Talofofo Beach is probably the best beach for beginners to practice on as you won't have to worry about washing out on shallow reef!
To be honest, if you want to learn to surf it's best you go somewhere like the Caribbean islands, Mexico or even California. Guam is mostly for experienced surfers. There are spots for beginners like Glass Breakwater Beach (near Apra Harbor) but as a beginner you will be disappointed.