When non metals form ions what ions do they tend to form?

Non-metals have a larger number of outer electrons called valence electrons. Therefore, it is easier for them to "steal" valance electrons from other elements with fewer valence electrons(usually metals) 

For example, Chlorine(Cl) has 7 ve-. When forming an ion it will steal one electron from a metal. and gain a charge of -1. Because the ion is NEGATIVELY charged, it will be called an anion. 

Nonmetals form anions.

Vocab:
cation: Paws-itively charged ions (get it? because its a CAT ion?) 
anion: negatively charged ions