Hi, i'm new in this great game, sorry if my questions are stupid but i'd like to know something more .....( when playing alone ) i've reached level 20 and 300.000 points xp but after ? Even if i gain new xp during the games they are not added to the previus xp so........i always have 300.000....and without new xp i can't take new abilities.....and after level 20 ?

You don't have the DLC, which can increase the level cap to 24. The highest level you can go is 20 in vanilla Brink.

...ok...i suppose DLC is a kind of....continuation o f brink...and,,,where can i have this DLC ?

You should be able to get it from your console's online content store or perhaps from Bethesda directly.

do tou mean Brinks agent of change DLC ?

Yep that's the one