"I am wearing a sock."
Translation:Εγώ φοράω μία κάλτσα.
It depends on the word's grammatical gender. Μία is for feminine words, ένα for neutral and ένας for masculine ones. There are no foolproof rules I'm afraid and the cases complicate things as the endings change so you have to learn every word with its gender... If you're making notes, write down every word with its (definite) article (they're shorter!).