Can you teach me the 'If' commands in Excel?

I need a formula that will give us a person’s age when we plug in their month, day and year of birth. Subtracting the current year from their birth year gives you an idea of how old they are, but it’s not an exact number unless you factor in the month and the day and I don’t know how to do that.
I’ve been wanting to learn the ‘if’ commands but just never got around to it. I’m ready now, Teach! Bring it on!

Thank you. : )