Buy the 2015 Old Farmer's Almanac!
Reporting
first-time watermelonYou can use the soil you describe but you probably will not have great results. Sand does not hold water; water filters through it. "Water"melon plants need quite a bit of water as they grow; if there is no soil to hold the water around the plant's roots, you will be watering all the time (almost literally). If you are going to buy regular potting soil, as you describe, ask at the place you buy it if they have compost. It usually is available in 20- to 40-pound or so bags. (You do not have to use fresh from the farm compost.) You can mix the compost with potting soil. How much?? Maybe 50-50. The goal is to have "soil" with ingredients that hold moisture. (This is desirable for almost any plant, certainly edibles. Many tropicals or types of plants that you see thriving around you in the sandy soil would not do well in compost because they like the dryness that sand enables.) As to the fence and hills, sure. Those decisions are secondary to the soil. And, by the way, sun. Do not plant the watermelon in shade. It needs about 8 hours of sun per day. We hope this helps. Good luck!

2015 Garden Calendar2015 Weather Watcher's Calendar2015 Recipes Calendar2015 Engagement Calendar 2015 Everyday Calendar2015 Country CalendarNew Year Cross StitchLobster Rope Doormats