go HERE and read .. This page is about wheel building from scratch but just the "Tensioning and truing" section, bout half way down, should give you a good idea what to do.
Notice that half the spokes are attached to the left side of the hub and half to the right side... and each spoke has an adjustment nipple where it goes into the rim. (Buy a spoke wrench that fits your spoke nipples at a bicycle shop)
If you _tighten a left-spoke_ it pulls that section of the rim to the left. _Loosening a right-spoke_ in that same area also lets the rim move towards the left.
So, to move a section of rim to the left, you tighten a few nearby left spokes and loosen a couple right spokes.. 1/4 or 1/2 a turn at a time.. tighten this one, loosen that one, etc..
The whole thing is confusing at first .. left right up and down tighten loosen .. it all sorta melts together..
but if all you need is minor adjustments and you're careful and take your time you can end up with a really straight wheel..