There are a lot of strategy in trading cryptocurrencies, the most common strategy is what we called "Buy low, Sell high" I think you already knew that. So you're wondering what coin you should buy? you can check the first 10 or 15 coins here in https://coinmarketcap.com/ and study every coin well and try to invest in it.

If you're looking for detailed strategy i can give you a link. I've read this blog last month ago and i think this is a big help it includes detailed strategy, entry points and exit points etc.
Here's the link: https://www.deepdotweb.com/2017/04/30/tutorial-altcoin-flipping-cryptocurrency-trading-strategies-always-win/