When I would true wheels back before I had real bike tools, what I would do is turn the bike upside down, take the tire, tube and rim strip off the rim, place the rim back in the frame to use the brake pads as centering guides and then use a flat screwdriver in the nipple ends through the spoke holes. Cheap spoke wrenches will tend to strip nipples but I never stripped one using a flat screwdriver in the nipple ends.

If you buy a spoke wrench, buy a name-brand one--DO NOT get the cheap generic ones at wal-mart or anywhere else!

For the actual truing, always do adjustments in 1/4-turn increments so you do not un-center the rim on the axle, and always loosen one spoke and then tighten the one next to it. Wherever the rim is pulling to one side, loosen the spoke pulling to that side 1/4 turn FIRST and then tighten the adjacent spoke pulling to the other side 1/4 turn SECOND. Spin-test the wheel and expect it to take a bit of time. You can get it darn-near perfect with no real bicycle tools and a bit of patience.

Also when spoke wrench shopping: spoke nipples come in at least four or five different sizes (widths).... The wrong size will either not fit on at all, or will strip the nipples. So you want to get a wrench that has at least 3 different typical widths on it (like the Pedro's), and then one of them will probably fit.