I've been teaching a bunch of SUP surfing classes recently on the Washington coast. Here's a list of common errors students routinely make and some solutions to those errors..
Not paddling straight towards the beach while trying to catch a wave. To best catch a wave you want to be faced perpendicular to the wave - or facing the beach. I've had students paddle diagnolly with a wave behind them. Sometimes it works, most often not.
Correction: When gaining speed to catch the wave, paddle straight towards the beach while watching the wave behind your shoulder. Learning to paddle straight and on one side allows you to focus on your speed, your position in catching the wave, and timing in catching the wave.
Many students stand facing the beach without paying attention to waves approaching from behind. Doing so not only prevents you from choosing waves as they approach you but also prevents you from watching out for other surfers.
The surf zone is a very active environment and can be dangerous if you're not paying attention. Always look behind you for other surfers, waves, or obstructions. I often stand sideways with the beach on one side and the horizon on the other while waiting for waves.
A common error is not gaining enough speed before the wave hits you while standing straight up. The wave will knock your board out below you if you're too slow or standing upright.
Solution: Start paddling 20' before the wave comes to you. Crank up your speed when it's 10' away then squat down low as it comes up behind you. As mentioned above, keep your eye on the wave when it's behind you and when surfing. Work on your forward stroke so you can effortly paddle fast and straight in the surf. I use short quick strokes with a wave fast approaching behind me. Short means taking the blade out at my feet or toes. Don't stop paddling until you're surfing.