Once you have purchased and received one of the Guardian packages, simply login to your account and login to your character. At the right of the screen is the Game Menu which lists your classes. Click/tap on the Guardian class to equip it. Once you equip your Guardian class, you will also have the Guardian armor equipped.
The Guardian armor is the default or 'base' armor for the Guardian class so you will always be wearing it when you have the Guardian class equipped. You can obtain more items by talking to the NPC Fortes inside the Guardian Tower.
You can obtain your other items by going to the Founder's Sanctuary in game. Everything else that comes with your Guardian package such as Dragon Crystals, bag slots and bank slots are automatically added to your account.