at no point do you want a big ball end on the tungsten, this lends to a very unstable arc and lack of directional control. this is a leftover from the bygone era of "heliarc" AC welding with pure (green band)tungsten, generally acheived by womping on the tungsten with a bunch of DCEP. if you're DC welding you'll want a sharp point, and with AC welding you'll want a conical point with a flat tip. there's obviously a lot more to it than that, but that should suffice if you're just starting out and need some general rules, that'll get you going.
-scott