In my opinion every time there is public news about altcoins (newspapaer articles etc), the price of altcoins altogether is affected. In march for example people speculated on the approval of the Bitcoin ETF and the prices went up like crazy. In August there had been a crash because there was talk of a new bitcoin coming out that should fix the problems of the old bitcoin chain. Recently the prices dropped again because apparently china was talking about banning ICOs. My guess is to to stay up to date with the news.
And if you want to buy cheap altcoins in order to make money, look for a coin which has a very low marketcap, the prices will multiply themselves quicker than if you bought for example a bitcoin which would have to reach the price of 8615$ to double its price as of today.
Another tip that I can give you, although it is not my favorite choice, is to try and spot pump and dump groups, but be careful how much you invest into those in the beginning since those pumps are over very fast (you have like 2 minutes or less to sell the right coin before everyone sells if you want to make any money). better start small and build up as you feel comfortable