Moisturizer goes on first then sunscreen. Apply the sunscreen 20 minutes before going out in the sun.
Here's what Paula Begone author of "Don't go to the cosmetic counter without me" says about using sunscreens.
http://www.cosmeticscop.com/learn/sun.asp?ID=163
I also use Clinique Repairwear day, SPF 15. I use Neutrogena Ultra Sheer Dry Touch with helioplex for biking, hiking etc, on all exposed skin. When I'm out all day I reapply it every couple of hours.
Paula Begone, and research, says that sun exposure is the cause of wrinkles so it's important to protect yourself at all times.
I wish I had payed attention to my grandmother 45 years ago when I did all that unprotected sun tanning. Now I'm paying the price.