Manuel,

1 ohm=1 volt for 1 amp. 1 ohm=2 volts for 2 amps.
0.1 ohm=0.1 volt for 1 amp.
I think you can do the math.
Question AC or DC. This may make a difference. How much current? Power Line? (110/220)?
What resolution? If you want to know; is load on (about 0.1 volts) or is load off (about 0 volts) then the PIC can measure directly with no amplifier. If you want to know 1.05 amps then you will need a amplifier! (0 to 0.1 volts on resistor---50x amplifier---0 to 5 volts to PIC.

rons