What exactly do you mean by some pimples? If it's really affecting you then please see a GP/dermatologist. They can give you advice and prescriptions for effective medication.
My advice:
-cleanse twice daily with a basic cleanser e.g. Cetaphil, Neutrogena Extra Gentle Cleanser (not one that is designed for pimples like Clearasil etc, as they tend to be too harsh)
-I know it sounds random, but spend some time in the sun (not heaps, you don't want to look like a lobster!). Also, salt water helps pimples to heal. So go to the beach!
-don't pick them or keep touching your face. I know its tempting but it won't help.
-go to the chemist and get an acne cream with benzyl peroxide, 2 or 5%. Put this on your pimples each night, but be warned it does bleach pillow cases and towels, so use white ones.
How long is it until your formal? What I've written above would probably take a few weeks to show results.