How do I pick a Alt-coin, what do I look for?

There are so many currencies and tokens on the coin market cap, so how do I know which one to HODL for short, medium and long terms?

For short term, this is mostly pump and dump coins , the usual users for this is the day traders. Your source for this is obviously the following;
Twitter
Facebook
Youtube
Telegram and etc.
This is a risky scenario, so you better know what you're doing else expect lost.

For Mid term coins, looks for in development with unique technology, read the white paper and there road map. Usually when it is listed on an exchange, the dates on the roadmap trigger pump so HOLD on those coins until the development comes to fruition.

For Long term coins, check the Github checkins , development team , marketing team and uniqueness of the problem they are trying to solve using Blockchain. Better yet stick to the top 20 on the coinmarket cap.

Hope this helps.