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Use a voltage regulator like LM7805. They been around for 30-40 years, and they have a large family of LM78XXX regulators and they work great for simple projects with a fixed voltage. You don't really need any resistors nor even capacitors (optional) despite that schematics shows caps. If you want a simple variable regulator go to the LM317 which needs two external resistors, but that allows you to set it to any voltage. If one resistor is a potentiometer, you can dial in the output voltage. Again the caps are optional for smoothing out ripples (only needed if your primary supply comes from AC, rectified, and from a transformer).
Yeah, but I have used them for those cheap transfomers before because most of the time a 6 volt 500 ma wall plug in is ungregulated and is at 7.5 volts open circuit and drops off as the load is added. It all depends, but I've used and designed dozens and possibly hundreds of power supplies. In reality, he probably needs nothing like someone said, and he can use the transformer directly. If he really wants to use five volts, he can make a crude 5v power source wth a large enough rated zener and a resistor in parallel to his load but he will dump current like crazy (it won't matter since he is not trying to conserve a battery). Or he can put diodes in series such as a low voltage schottky (Germanium diodes) 0.22 volts plus a silicon diode 0.77 voltage then he'll get his 1 volt drop provided the diodes are rated for at least his load. However, like I said, I doubt his Wal-mart transformer is regulated and all he will do is drop a volt not regulate voltage. He could build his own regulator using a Zener as reference at the gate of an N channel Mosfet, and those transistor generally have only a 0.8 volt drop between source and drain. Anyhow, there are 100 different ways to do it, but I think the simplest solution is just a LM7805 because I am sure the transformer is not at 6 volts.
