How can I tell my boss that I can't work for 10 days?

I got hired at a Ben and Jerry’s which is a job that gets 100 CVs a week. The owner just hired people and I got hired under different circumstances. I was in his store giving my CV and he liked my personality, and other variables like the weather and his mood influenced him to hire me on the spot. This was last week. I’m supposed to go in for training tomorrow, but I won’t be available for 10 days, because I have a summer class for 6 days and then I as planning a vacation in Orlando with my best friend, who I’m splitting up with in the fall. This trip is very important to me. It’s gonna be 4 days.

So how do I tell my boss that I can’t work for 10 days starting this wednesday?