Macedonian denars (MKD) come in 10-, 50-, 100-, 500-, 1000- and 5000-denar notes, and one-, two-, five-, 10- and 50- denar coins. Taxi drivers hate it when you pay with a 1000-denar note, and may make you go into a shop to make change. Euros are generally accepted – some hotels quote euro rates, but it's always possible (and usually beneficial) to pay in denar.
Macedonian exchange offices (menuvačnici) work commission-free. ATMs are widespread. Credit cards can often be used in larger cities, but you can't really rely on then outside Skopje. Avoid travellers cheques altogether.