Translation:The girl is standing next to a red car.
See my other reply to your similar question about the building above a/one river.
In Hungarian the context defines clearly whether it is a numeral or an indefintie article. In this case the "egy" is the latter, so you must use "a". (I am aware that sometimes English speakers use "one" as indefinite particle [like "you're one fine gentleman, sir"] but this is not a "proper" standard English.