That's the beauty of our VPSes: you can't! The main CPU in the server is divided equally among each VPS using a “fair share” algorithm. This means that if two or more VPSes try to use the same core at the same time, the system divides their usage equally so that everyone gets a turn. This is an extremely fast process – so fast that you won't even notice your CPU time is being divided among other users.
If you happen to use all of your available CPU power, your VPS will simply slow down until you relax your load. If you happen to use all of your available memory, your VPS may crash – just like a real server.
Not all VPS providers work this way. Some providers take a “free for all” approach, which can lead to other VPSes stealing your resources, which can be a disaster.