Some people before have said some of this, but here's how I started. Equal distribution of weight. Look at end of rail. Reaching down for your skis a little can help to start, but makes it look bad. MAKE SURE YOU GET THE RAIL UNDER YOUR BOOTS. If you are on your nose or tail AT ALL and you will come off until you can nosepress and tailpress. And here's the biggest one for me, COMMIT. Just go for it. I got started skiing late, and my friends were already in park, so I kind of got forced to, but just do it. I just threw myself at whatever was there as long as it wasn't too big, and just tried it. Just believe you can do it and try it. I did my first rail my 3rd or 4th time skiing (it was a double barrel that was practically a box) but just go for it. Commit and believe you can do it.
hope i helped good luck