This article should not be considered financial advice. You should consult with professional to determine what may be best for your individual needs.
There were more than 900 cryptocurrencies available over the internet as of 11 July 20, how do we evaluate which ones are worth studying and buying?
Development
One of the most important thing is whether there is a good development team behind the cryptocurrency and how active it is. Search for the history of the developers, what projects did they finish in the past and so on.
The best way to measure the development activity is via github.com. Nowadays almost every developer uses Git (version control system for tracking changes in computer files and coordinating work on those files among multiple people). GitHub is a web-based Git. 
Market capitalization
Coins with low market capitalization can be easily manipulated by people planning to organize pumps and dumps tricks. Coins with a larger market capitalization will be harder to be manipulate as it will require a substantially larger capital base to manipulate the price. Lower market cap coins have greater potential to grow. Thing about the market cap, whether the coin is overpriced or not, you need to use your intuition here. Maybe half a billion dollar market cap for a coin that doesn't do anything yet and won't be usable for a year is too much.
When to buy?
I am not a trader, i am hodler, but i want to get in a good position, that's why i use some other short therm price indicators. I use google trends to determine how many people are searching for a particular keyword. Here is an example:
https://trends.google.com/trends/explore?date=now%207-d&q=ethereum,monero
The more the currency is searched in google, the more attention it gets and that usually means higher prices, don't rely on this indicator heavily though. I know many people say that technical analysis is nonsense, but sometimes it might give you a bit more information to decide. I don't rely on technical analysis very much, i use only two simple indicators. RSI which measures oversold and overbought conditions and MACD which helps me to recognize a trend reversal.
Other tips
These are very useful tips for evaluating altcoins from John Jeffrey Mardlin, Community and Customer Support at Coinbase:
Does it have a community forming around it? Are they welcoming? Are they vocal?
Number of subscribers to that cryptos subreddit
Number of twitter followers, twitter mentions, facebook likes, etc.
What is the coin issuance scheme, and how heavily does it favour early adopters?
How does that compare to other successful coins?
Does it have a marketing gimmick, like being the 'official crypto' of a specific country, or a good meme?
Has it peaked yet?