how do you true them?

With a spoke wrench.

And look here: http://www.unicyclist.com/forums/showthread.php?t=41748

What’s truing???

Truing a wheel is making the wheel straight. When you turn your unicycle upside down and spin the wheel there should be little to no lateral movement (the wheel shouldnt wobble side to side). If it does the wheel needs to be trued. If you do not wish to mess with the spoke tension, take it to a local bike shop and have them true the wheel for you.