W-4 and College Student

My children are 18 and 23 and I still claim them as dependents. What should I put on my W-4 as the head of household? What happens when I'm not claiming them as dependents. What would I put as a single person on my W-4?

 

When my 18 year old becomes a full time college student, should I be claiming the opportunity credit on my taxes if she owes money or what if she doesn't owe?