Numbers (1-20)

1 Uno
2 Dos
3 Tres
4 Cuatro
5 Cinco
6 Seis
7 Seite
8 Ocho
9 Nuevo
10 Diez
11 Once
12 Doce
13 Trece
14 Catorce
15 Quince
16 Dieciseis
17 Diecisiete
18 Dieciocho
19 Diecinueve
20 Veinte

That's pretty much it as the numbers 1-20 go, but one little point of grammar:

With masculine nouns 'Uno' changes to 'un' - Un chico - a (1) boy
With femanine nouns 'Uno' changes to una - Una Chica - a (1) girl