My brakes make a grinding noise only when I reverse oh a slant or when I just start up my car and drive it, it'll make a grinding noise, but after I drive a little the grinding noise dissappears. Is it because moisture or is it my brakes wearing out or my rotors?

Could be moisture, or misaligned pad.
But I'll check my brake pads if I were you.

ditto

No easy way of doing this, get ya wheels off and check your pad
thickness through the aperture in the back of the calipers, also
check for scoring on your rotors. Are you hearing a grinding or
squeeling? Could be the brake wear clips rubbing the rotors.
I had some noise on reverse too but copper grease on the
caliper pad assembly cured that....

Jon

Get your brakes checked. most likely it is just brake dust or a pebble in between the pad and the rotor, but better to be safe than sorry.