Numbers are one of the first things to learn in any language, and German has a few quirks to watch for. This list goes in counting order, so you can learn it from the top.
Counting
From eins to zwölf the numbers are all separate words. From thirteen they follow a pattern: dreizehn, vierzehn, fünfzehn, which is the unit plus -zehn. Two to watch are sechzehn and siebzehn, which lose a letter (sechs and sieben become sech- and sieb-).
The tens end in -zig: zwanzig, dreißig, vierzig, fünfzig. Thirty is dreißig, not dreizig.
The numbers turn round
From 21, German says the units first, then the tens, joined with und: einundzwanzig is literally “one and twenty”. It is the same in 22 (zweiundzwanzig) and every number up to 99, and it takes some practice.
Also note that eins is what you say when you count, but before a noun it loses the -s: ein Euro, ein Buch.
Saying a price
A price such as 3,50 € is read drei Euro fünfzig, with no word for the cents. In German writing, the comma is the decimal mark and a full stop separates thousands, the opposite of English. The word Euro does not take an -s after a number: fünf Euro, not fünf Euros.
Wie viel kostet das?
How much is that?
Das macht drei Euro fünfzig.
That comes to 3.50 euros.
Paying
In a shop you may be asked whether you will pay bar (cash) or with a Karte (card). It is worth keeping some cash, as smaller places do not always take cards.
Zahlen Sie bar oder mit Karte?
Are you paying cash or by card?