If you have a 12V wallwart, use a 7805 to get 5V, that is the easiest way to go. If you only use 100mA, there is little loss in the 7805, but you should still use a small heatsink anyway.
There are several small DC-DC switch mode regulators that can be used, but you need more components outside the chip. In addition, these regulators are normally only available in SMD packages. This mean they can be difficult to solder by hand, if you are a starter in electronics.