I am not a full time trader yet but I would say only invest what you're willing to lose. Selecting investments would mean you need to look at the project, whitepaper, figures, road map, team etc. Some coins are for hodling and some can be used for day trading. For day trading keep an eye on whats being released onto exchanges, watch some specifinc ones everyday and see  how it rises up and down. Learn how to use the graphs to your advantage (something I have not learnt yet)