well for a more in depth answer......
get yourself a tape measure
10mm and either a 12mm or 13mm wrench
ok, straighten out your handlebars, now measure the tires center to center in the front, now do the same on the rear. if the measurement is larger on the front then it is "towed out" if the measurement on the back is larger then it is "towed in". you want to get this as close to perfect as possible (within 1/16th if possible).
sometimes you can see that a wheel is pointed one way or the other after straightening the bars, if this is the case that is the side that you want to adjust. if they are even then you want to adjust both evenly.
another thing to check is make sure that you don't have a brake dragging or anything as that can make it pull as well.