Glad to see a new Android User
Procedure to flash a Rom on any device is... First, you'll have to acquire root. So go ahead and look for a good rooting method for your Galaxy S4. Next you will need to Install a custom recovery, in some cases people install the recovery first then root their device. You'll have to figure that out. Then just simply download a Rom you like. Backup ur current Rom, wipe data and flash it.