Really try to lean and dig your edge in to properly "grind" it, hard as you can.
Actually NO I'm joking, try to NOT do that. Do FLAT rails first so you get an even balance left and right worked out. I think that's the best tip possible for a complete or near rail-virgin. So many first timers get rocked on the down rails. You'll learn to keep your torso over your feet as you practice, and adjust for steeper rails, kinks, and so on. Boxes are easier but way less slick. If you learn rails first you will hustle it on boxes.