In short: you could lose it all. Be careful with margin, decades ago I almost "lost it all" but fortunately had some help from my folks and made it through the rough times, with lessons.
More recently I've tried Poloniex's margin exchange; it seems to track differently than their regular exchange -- it seems there's some manipulation, perhaps. Being on the lending side made more sense (using the "Poloniex Lending Bot" which you can find with a search), but I stopped that when they had a glitch and their site was down for a day or so, I think that was back around July.
Anyway, a good goal is to never go more than halfway into margin. For instance, if an exchange allows 10x margin, don't use more than 5x -- and, I'd caution you not to use more than 2x, to start with. Get your feet wet first, before the water covers your nose! :)
Good luck!
Thanks for the advice. Yeah, I am wary/scared that I might set some number wrong and inadvertently raise my risk higher. Shorting the biggest bull market in history is usually a bad idea. :p If I was going to leverage, it would be on something like a calendar event on some development road map the next day where the likelihood of a drop in that time is low and the likelihood of a run up is significant within 24 hours.
People shouldn't be margin trading unless you have the funds to back them. lol, With Crypto, you short or Margin trade and you can lose quite a lot.