Yeah, definately get the same 2 tires on the rears and the same 2 tires on the front.
But since money seems to be the issue here, *pause* damn
Just get 2 tires with the same amount of tread and they obviously have to be the same size to fit the rim.
You should be ok. Yeah, tell your mom that you need to be more independent and get your licence so that you can 1) get a job 2) prove that you can be independant 3) You can stop bothering her for $$$ to pay for stuff.
My sister is 16 and she has a waitress job and is making bank, like $50-$80 per day on the weekends only.