"You don't need a taxi."

Marion, it's from the verb necesitar - to need.

If you remember your present tense verb conjugations:

yo necesito - I need

tú necesitas - you need

él necesita - he needs

usted necesita - you need

ella necesita - She needs

ellos necesitan - they need

So, tú no necesitas un taxi - you don't need a taxi.

It could also have been usted no necesita un taxi, 'you don't need a taxi' using the Usted you formal form.

Just don't forget to match the conjugations correctly:

tú necesitas, usted necesita

You need to add the Tú.