You need to cover most of the exchanges because sometimes the opportunities are not in a single platform. I'd go with Bittrex, Poloniex and probably HitBTC as well. I can tell you that the basic rule is to sell high and buy low, and that you should never sell out of fear and also never buy something that is involved is some pump and dump scheme, It always end up badly and the market could crash. It's better to invest into projects that will actually deliver something.