There are a number of supplies you need to have with you when you play group content. I will touch on all of them on this page.
Purple CP150+ food (Increases max Health, Stamina and Magicka)
As a tank we use all three stats. This purple food gives us the maximum total stats we can get from a food item. This is also the reason we choose to go with purple food.
Essence of Health CP150+ (Restores Health, Stamina and Magicka)
These potions restore all three of our resources and will give us 20% recovery on all three of our resources for the entire potion cooldown. These potions will give us the most value as a tank.
Ingredients needed to craft: Bugloss + Columbine + Mountain Flower
Always make sure to have enough soul gems on you. You will need them if you die or if you need to resurrect other players. Also, you will need them to recharge the enchantment on your weapons.
Also make sure to have quite some repair kits on you. If your team is trying a new encounter and you die a lot your armor can get damaged. Repair kits are an easy way to solve that problem.