So you want to Dodge Roll without having to wait for giant trains trying to splatter you to get the mini tutorial to come up, am I right?
To perform a Dodge Roll, you just need to hold the aim button, then press the direction you want to roll and press the action button (A on 360, X on PS3), or click down on the left analog stick.
If you continue to hold the aim button, you will stay prone on the ground aiming your weapon, which can be handy in some situations. While in prone position, you can hold left or right and press the action button again to roll to either side again.
