I would suggest a soil test to start

A soil test is going to tell you that you have sand with no nutrients. Let's get some grass growing first.

St Augustine loves a little shade. Bermuda, the other popular grass in FL, will not grow in any shade. For full sun the best one to get is the Floratam variety.

There are a couple tricks to growing St Augustine. First is to mow it at the mower's highest setting. Let it get up to 5 or 6 inches tall before mowing it back down to 4. Second, it absolutely must have deep water every so often. When temps are 90 or above, you will have to water a full inch every 5-7 days in full sun. This time of year you might get away with watering every 3-4 weeks (always a full inch as measured by cat food or tuna cans).

Forget about worms. They might come later or might not. The best thing to improve your sand is to use organic fertilizer. If you really have fine, white, sugar sand, then any amount of compost you can afford, up to 1 cubic yard per 1,000 square feet, would be helpful. My favorite organic fertilizer is alfalfa pellets (rabbit chow). Apply at 20 pounds per 1,000 square feet as often as you want to or can afford. Grain type organic fertilizers take 3 full weeks to see any improvement. They are called "slow release" but that's sort of a misnomer. They release fast but not to the plants. The fertilizer feeds the soil microbes and they feed the plants. Being a biological process it takes time. Over time your white sand will appear almost black. Don't be fooled into thinking your sand is changing. What's happening is the microbes and grass are leaving behind organic matter that will help to hold moisture and provide other benefits to your soil.

As for the snails, find some cheap ammonium sulfate fertilizer and scatter that around. Ammonium sulfate looks like rock salt and has exactly the same effect on snails and slugs. They bubble up and die. We had a big infestation back in the 90s. I used that stuff (before I became organic) and we haven't seen a snail since. Incredible!!