I think you could start by asking yourself why do i want to buy altcoins? Pending on the answer you would have to look at different thing. What is the risk i am willing to take what is my horizon? Etc.

If you are in it for the long term look at established coins that have a good history. If you are looking yo get rich quickly go with the flow.........and hope for the best. The selection of voins is the last thing to do and also have a plan and stick to it soemotions do not get the better of you.