And indeed, as others have previously said, I wouldn't worry so much if you're just starting out! But I'd still try to follow the CAA guidelines for formatting, etc.
I don't know; I think it depends on the institution and networking opportunities, but I'm generally wary of unpaid positions. You can always negotiate (a friend of mine was offered an internship, but he convinced them to at least pay him $8/hr, which in NYC is not much).
Also, I'd probably target internships that give you more responsibility rather than bitch work.
My two cents.
I would follow the CV formatting guidelines for art historians on the College Art Association website: http://www.collegeart.org/guidelines/arthistcv
Mine is ordered as follows:
Education
Professional Experience (you can list other jobs here, but I'd probably only focus on what is relevant)
Awards/Fellowships/Grants
Publications
Presentations
Professional Affiliations
Languages