Theres really no way to guarantee that you wont lose money but I would say that following the basic principles of not being too greedy and not being scared to buy in are good ways to ensure a better success rate.  The jist of it that you should leave emotions out of trading.  When it comes to trusted sites, i use GDAX and i've used poloniex before.  Both are good exchanges but gdax provides usd and fiat trading.