It can be whatever you like. Just make sure it's your own photo, not something from the internet.
You can photograph cars with foreign plates, exotic cars, rare cars (new or old), custom plates (custom text or ones with repeating numbers/letters), old plates, whatever.
There isn't any "prizes". You can get likes on your photos, your photo may become "Photo of the day" for the country the plate is from if it's the most liked for that day, or "Photo of the month" if it's the most liked in the month!
Just find what interests you and do that.