Before a match, you select your loadout and map various weapons to certain button inputs. Once in-game, most actions have a tap, hold, and release action. Some weapons let you strafe while holding while other might root you in place. Some actions are tap-only. You can dodge roll to get invincibility frames using the dash button (LB/Tab by default). You can also parry with the parry button (RB/R key by default) to gain a few frames of invincibility and temporarily stun enemies that melee attack you.
I have a How to Play menu on a development branch** but it hasn’t made it to the demo yet since I’m working on other things at the moment, but hopefully that helps you for now!
**EDIT: The latest demo now includes the How to Play menu mentioned above.