Well maybe a little tutorial first
Rom=the firmware on your phone
StockRom=official (Samsung provided) firmware
CustomRom= unofficial firmware (rooted phones only)
Recovery= it's a surface in which u can install. Zip files and do other stuff like wiping dalvik cache etc. You have to differ between stock-/customRecovery
To install zip files you need a custom recovery like clockworkmod (cwm)
(to) flash something= that just means to install zip files via custom recovery
First thing you need to do is installing a custom recovery , depending of your phone model you can eventually use rom manager from playstore (gt-i9100).
If you have another model you may have to find another way to install cwm as I have no idea if it works on other models as well.
If you installed cwm successfully you now have to search for a rom which is fitting to your phone model, be careful, as flashing of a wrong rom can kill your phone forever.
To start the recovery you have to start your phone while holding the power, home and volume up button pressed. Once there it's a good idea to make a backup before flashing something.
If you have installed cwm through rom manager and have still stock rom, you only can start cwm by using rom manager.
Better wipe every data before flashing a new Rom otherwise you may experience a boot loop.
Alright guess that should be all for now, hope it'll help
Sent from my GT-i9100 using xda app-developers app