You have what is known as a "bootloop". First thing to try would be a clearing of cache and dalvik cache in recovery mode. In standard recovery you can clear cache direct from the first menu, but I'm not sure if dalvik cache is available. If it is it is under "advanced" from main menu.

To get to download mode is power off>complete shutdown>home + volume up (press and hold these) and lastly power to press and hold (three keys - all at the same time, but power lastley). If it won't turn off pull the battery, wait ten seconds put battery in and try to get into recovery as above. This may well work.

If it does not work you'd have to flash your stock ROM using Odin from a PC. Try the wipes first, if it doesn't work get back to us in this thread.