If you are near a cliff, chase the tarantula off of the cliff. Then run to the nearest ramp and bring out your net. Go to nearly the bottom and the tarantula should run into the cliff. then take a step back and swing your net.This should catch you the tarantula.
You should always keep your net out. Don't wait till you see one to whip out your net. Cause it'll attack, those weird things...and do what the person above my post said. *whispers* it really does work. :3