Spells that require Concentration can only be maintained one at a time. If you are concentrating on one spell and cast another, the first spells effect end immediately, unless the spells description says otherwise.
You may at any time end the concentration of a spell, without costing an action.
If you are incapacitated or killed, you lose concentration.
Concentration Check
When you are concentrating on a spell and take damage, you make a Concentration check, which is a d20 + Constitution modifier and you need to overcome a DC equals to 10 or half the damage taken, whichever is highest.