When it comes to skimboarding, the time and location play a very big part.
If you're just starting out on a flatland board, then you definitely want to find somewhere as flat as possible where the water washes over the sand like this.
The perfect time for flatland skimboarding is around low or high tide because that's when the water creates that wet sand you need to skim on. As for wave riding, the best time is usually high tide. You want to look for some onshore waves like this.
You'll want to start on something small. You can still learn how to wave ride on white water.