hey all...first time user here looking for some advice

thanks to anyone in advance for replying to my post. i appreciate it...i have been playing online poker for about 4.5 months...my question mainly is in reference to bankroll management, and more specifically, what limits I should play at with a given bankroll...No Limit poker 20x buy-in? or more...do I need to 20k to play 1k NL? or 40k..etc etc... any help would be great guys thanks!