So for your first rail you are going to want to chose something fairly low to the ground, with a classic style lip. (Easier than skate style at first if you are just starting,) Try and find a rail with the lip as high as the rail.
1. Radness call-out first, obviousely. Give a solid "HEY CHECK ME OUT SLIDING THIS RAIL!"
2. You have to drop in confident, if you drop in thinking you are only going to slide half the rail, you probably will. Come in with more speed than you think you need and be ready to slide the whole thing.
3. Give a solid pop off the lip. This will help you get on the rail smoothly, and you will have to do this once you start hitting rails that aren't ride-on. Don't try and turn into the rail before you are in the air.
4. Try and land as lightly as possible and imagine you just ran and jumped onto a skateboard. Land with your weight balanced, not leaning forward or backward. As soon as you land on the rail and are stable for a moment, look at the end of the rail, and try to move as little as possible. The last point sounds weird but I find it works well on the simpler rails.
5. Land, ride away like a boss.
6. CLAIM HARD