I read about the nitrogen cycle, the fishless cycle, and the fish-in cycle. Which one am I suppose to do first? I plan on getting a 20 gallon tank pretty soon with some cories, neons, and a few other fish.
They all refer to the same thing. Growing a colony of ammonia and nitrite consuming bacteria the vast majority of which will be inside your filter.
There are a number of ways to achieve this but I'd suggest the fishless cycle as a fish in cycle can harm your stock.
But before you start I'd recommend you start looking for an external filter which filters from 20gal and up. I'd recommend and ehiem and strongly advise against a tetratec. And also if you have bought a juwel aquarium, that you take that internal filter out as it's really not worth the space.
I wouldnt worry about going out and spending money on an external for a 20 gal, lots of cheaper good internal would be fine on that sized tank such as a fluval 4+ or fluval U4.
Why do you strongly advise against the tetratecs londonguyj? I've found them to be fantastic filters, I do agree about the eheims being a great filter though.