What guard should you use in mma?

Answer 1

This is an extremely broad question, but I'll try to answer.

On the ground these are the main guards (there are more)

There is full guard (both legs wrapped around the opponent) if your on your back there is where you want to be (in general)

Half guard is where your legs will be wrapped around one of your opponents while your on your back, this position doesn't allow as many submissions as full guard, but there are sweeps and if you get your hips out, you can take your opponents back.

Butterfly guard is where you will be sitting down and the opponent will be sitting on top of your legs, your knees will be between you and him. This position is useful for sweeps and has some submissions.

I will also cover a few guards for striking

Crazy Monkey is a system that was made for kickboxing. You deflect punches with your elbows/forearms and counter with punches. For an example in the UFC watch Rampage Jackson

The Philly Shell is from boxing. when using it your lead hand will be lower than your rear, generally around your belly button. You must utilize a lot of head movement for it to be effective. You can also deflect punches off your lead shoulder but in mma its risky because of the small gloves. The low lead hand of the Philly Shell also leaves you open for head kicks. In the UFC it is used effectively by Rashad Evans.

There is also the traditional muay thai guard that mainly utilizes parries for punches. To defend against a kick in muay thai raise your leg in the air and turn it towards your opponents kicking leg.

In boxing to avoid punches they utilize slips, parries, ducks, shoulder rolls and more. They also catch punches off the shoulder to minimize damage.

In all of the striking guards and important part of your defence should be to get off at an angle to your opponent, this will allow you to be safe from his attacks while you are executing yours.

I hope this helps