need just 1 tile and tutorial with it.

[kylixer]

I need a black tile so i can make my rooms smaler than 15x20, or oddly shaped, and once i get it how can i add it to all my tilesets?

[blueXx]

if you'll leave a tile blank it will auto-become black
same goes for if you make a tile white.. which is the same thing..

[Tron]

Yeah, auto-black is nice, but the bottom of rooms smaller than 15x20 or  20x15 or whatever, is still passable. You can walk right under it, meaning that the bottom of your room would have to be on the bottom edge of the map. See?
And once you learn how to use walls and such, it's quite easy. I suggest dwarra's House Interior Tutorial

[blueXx]

you can block black from being passable, that's for starters
and yeah walls are 100% important, but that's not what he asked for so please stick to the problem