Currencies are a compact, high-yield geography topic. There are fewer than a hundred and eighty in circulation, a great many countries share one, and the questions are almost always of a single form: which country uses this, or what does this country use.
The groups do the work
Learning currencies country by country is inefficient, because they cluster. The euro covers twenty countries; the CFA franc covers fourteen across West and Central Africa; the dollar, in various national forms, covers more than twenty; the peso, the rupee, the dinar, the krona and the shilling each cover a recognisable regional family. Learn the families and the exceptions, and most of the map is covered.
The exceptions are where the marks are — the countries whose currency has a name nobody else uses. The kwanza, the ngultrum, the lempira, the quetzal, the tögrög, the vatu and the kyat are all reliable setter favourites for exactly that reason.
Subunits, symbols and the history
The second question shape asks about subunits — how many of what make up a unit — which is where the more obscure names live. The third asks about symbols, and the fourth about currencies that no longer exist: the countries that adopted the euro and what they used before, the mark, the franc, the lira, the drachma, the peseta and the escudo.
Related history worth having: German hyperinflation, Bretton Woods, the gold standard, and the stone money of Yap, which is asked about surprisingly often as the answer to "what is the strangest thing ever used as currency". Wider geography questions are here.