Before you do anything, check you can get into Download mode using the 3 button method? If so you are pretty much unbrickable, so have a go.
- Put your phone in download mode, dont plug it in yet (Download mode can be achieved by pressing all at once: power, volume down and home key)
- Unrar the JM5 file to find 3 files inside.
- Open Odin
- Load 512.pit into the pit section
- Load CODE into the PDA, MODEM into the Phone and CSC into the CSC
- Don’t tick re-partition
- Plug the phone in (while its in download mode), wait till Odin sees it
- Click start
Once you have done this, you should install some apps (paid and free), get your contacts all synced up (I use google for this). At this point, you should look at a lag fix.