you shouldn't listen to people who says that cryptocurrency will go up in the future, you should be only listening to yourself when it comes to investing.

Altcoins is a very  large term and when you say this term you should be referring to something more specific.
Each altcoin has it's own unique features and you should decide on a specific one based on how potential you think it has and what is the success rate you think it might has.
When investing in altcoins always consider the risk that it might involve as most of the altcoins tend to go up and down really frequently and in an aggressive way.