I heard that theres some kind of monthly fee after you reach lvl 20. can someone explain this.

After level 20 is reached a $10 fee per week is required in order to continue advancing levels

It is $9.95 per *MONTH*, not week, and you can also subscribe with a buy 3 months, get the 4th free deal.

In other words for all those people out there who dont have time to read it all here is 2 words for you.

It's usless.

There are a lot of things to do while level 20 and under.  We have some people who have played free for 3 years.

so forr instance im lvl 20; I decide to pay for a month. In that month I make it to level 25 and then stop paying: do I keep level 25 or go back down to 20?

Speaking as a Games Developer, you may just be onto an idea here..

No, once you pay and go over level 20, you have to be a current subscriber to play.  Otherwise someone could pay for a month, power level up to level 100 and then stay as a free-to-play level 100 forever.