You'll probably need(8x20ma=240ma per group) and atleast a hfe of (240/20=12) so a pn2222 will work with ic max @600ma, and hfe at 150ma is 100min( so even 5ma should turn the transistor on)
all the led negatives will go to the collector, the emmiter to the ground, and the base thru a resistor(220ohm-1k) to the arduino ouput
ofcourse all the leds will need resistors don't forget