thanks very much, i drawed a little schematic with Vin, Vout and 2 resistors, the Vout is 5v and the Vin is 12v, i was still missing one variable on the equation 5/12=R2/(R2+R1) i decided that R2=1000ohms, considering that resistors have a limitted wattage they can handle, and the higher the ohms are, the lower the wattage is.
which means that R1 needs to be 1400 ohms.
i checked just to make sure the values are ok. I = 12/2400 = 0.005,
Vr2 = 0.005*1000 = 5
thanks 