Brad, KD, bluedog and others have given you excellent advice regarding clothing, so I'll only add the following 2 bits of advice -
1-Google hypothermia and learn what it is and how to recognize it and deal with it - make sure all in your party can do the same.
2-Nothing you can carry in your pack can replace that piece of equipment sitting between your ears. Use common sense and your instincts. The mountain will always be there - you can always return to climb another day if conditions are'nt right. I have turned back many times and never regretted it. When you finally summit it makes it that much sweeter.