Start with ride on boxes then progress to boxes you need to jump on to and then go to rails when you comfortable with that. Anyway, some pointers that may help.
1) You don't need much speed for small boxes/rails bought it feels smoother/easier with more speed than less. Also, the faster you're going the less time you've got on the rail for thing to go wrong.
2) As you get to the rail do a gentle pop THEN rotate. If you start turning/rotating your skis while you're still on the ground then you will not stay on the box/rail (at best).
3) Commitment! Really try and push/weight up the ski thats downhill/forward when you land on the rail. I bet if you watch people that are falling while learning on rails almost all of them have the feet slide out downhill from under them. This is because they don't have enough weight on their front ski when they land. You'll hardly ever see someone fall because they've got too much weight on the front ski. A wide stance will help but won't stop you from crashing if you put your weight on your uphill/back ski.
4) Once you're on the rail/box look for the end of it.
Thats the main stuff IMO. Just focus on getting it to work first before worrying about style. Once you've got the hang of it think about your stance a bit more. My sister would approach rails/boxes with a wide stance but once she'd popped she'd bring her feet back close together in the air. I told her to think about pulling her legs up rather than jumping so much and this seemed to help.
If you're having problems just ask someone who knows what they're doing....you're a girl.... skiing park.... I can pretty much guarantee that any guy there would help you out.