Like the message said, your going to have to install/flash a custom ROM on to your phone. Did you flash a custom recovery (CWM or TWRP) to your phone when you rooted it? If you did it should be pretty simple to flash a custom ROM to your device. I found a site that you can look at that explains what a ROM is and how to go about installing one for the Galaxy Y. It also has a list of popular ROMs for your device. As long as you follow the instructions closely you shouldn’t have a problem but always make sure to backup your important data before attempting to flash a ROM.

http://samsungmobileclub.com/top-10-custom-roms-for-samsung-galaxy-y-gt-s5360/