To block using a shield in Tribes of Midgard, you will need to press and hold either shift or LT, or L1.
Doing so will cause your character to hold their shield, which can help to effectively block all incoming damage for monsters or giants in the game.
You will also need to block a total of 25 attacks in 10 seconds to unlock the sentinel class in the game.
That said, you can accomplish this feat by quickly pressing the shield button when being attack by multiple foes until you you complete the class requirement in the game.
Once unlocked, you will be able to enjoy a class whose whole point revolves around using shields for both tanking damage and killing enemies in the game.
However, if you don't have a shield yet for whatever reason, you can always craft one via the armorer NPC for a few starting resources. If you want stronger shields, you will need to upgrade the armorer.
You can also find shields sometimes as items via camp/hideout chests and when you break them open. That being said, once you have a shield, you will be able to block like a true Viking warrior!
Check out some more Tribes of Midgard guides at Pro Game Guides!