All good advice above. A few points previously mentioned that I'd really stress..
1. Stay low and keep the knees bent. Biggest time I see people come out on an edge is when they are learning and standing too straight up or getting too tense.
2. If you're on box or rail and you're coming off the side of it, don't fight it, just come off and hit it again.
3. When you're on the box/rail, don't look down at your feet, look at the end of the obstacle. Don't go too slow either (as said above)
4. This may be personal opinion but if you're doing a ride-on box and you decide to boardslide it, don't just shift the board when you come to it, hop into it. That's another thing I see a lot, people trying to set up for the boardslide too early. The natural tendency on a snowboard when you are moving on-snow and the board is no longer pointing down the fall line is that you want to be on an edge. If you hop into it (I say hop as in both feet because a specific ollie isn't really necessary at this point), you can land flat based. I know there's threads here of people building balance beams for off-slope practice so you may want to look into doing something like that if you have stuff laying around you can build one with. It would probably be a huge learning aide when getting on rails from the side, but I learned the hard way (..CLANK!)
5. Take this one with a grain of salt because it's more comfortable for me but may not be for you. I find it easier to slide a box or a rail if it's directly under my foot so I usually do more nose slides than I do boardslides. I think it looks better as well and I feel like I have a ton more control when it's directly under my foot.
Anyways, good luck and have fun. You're gonna fall, but just like the first days on a board don't let it get to you.