"I want a motorbike."
Translation:Θέλω μία μοτοσικλέτα.
As you have already noted I'm sure, Greek uses grammatical genders...some words are called masculine, feminine or neuter.
The masculine nouns use some for of "ένας", the feminine use some form of "μία" and the neuter nouns use some form of "ένα". These are explained from the very first Skills and you should read the Tips ¬es from earlier skills for a better review. They are found at the start of the lesson where it says TIPS.
Or go here for all of them: