There are many altcoins in poloniex.com and yobit.net.
stop using Yobit, it is a bad place and if you get caught into the fake pumping hype they put out there you will lose a lot of money.
Especially ZEC, DASH and ETH have good value.
good value? what is exactly good about it?
the price is higher than others and that is all you can say about their "value" and at this point DASH and ETH are in a mini bubble which makes their value bad. and ZEC is generally not such a good coin in my opinion.
How can we know the altcoins that are at low levels and that are going to rise in the future? Does this have a formula?
you should look for potential in these coins. check their announcement, try to learn some basic stuff about these things and also about trading. it is a bit hard and needs a learning curve but you'll get there.
What are you paying attention to when investing in any altcoin?
the potential that I told you.
also news and hypes before they get pumped