how do i get the os back on my galaxy s3?

[AC Question]

cousin thinks he knows everything and tried to do a basic wipe. well he wiped my os so now i cant get into my phone at all it just stays stuck on samsung logo when i turn it on. when i go into download mode and push up for custom download, this is what i get: Odin mode (in red), product name: sgh t-999, custom binary download: yes (2 counts), current binary: custom, system status: custom, qualcomm secureboot: enable.
then it says downloading... do not turn off target!! and stays stuck.

 

[jasonandrews25788]

i think you flash the wrong rom files, which model do you have?