Hello i have a noob question:
to stake your coins you need to have the computer ON, am i right?
Yes, connected to internet also open wallet and unlocked (encrypt wallet) back up wallet.dat too
I am not sure if I understood that "dynamic POS" right. The block payout (where are we currently, 20TROn a block?) occurs, regardless what amount of TRON you hold in your wallet, is that correct? The coins just need to reach maturity?
Thanks in advance.

Yes that is correct. All you need is mature coins to be able to stake those 20 Tron blocks.