This one is pretty simple. For portfolios, follow the directions on the applications. Go over your work with your art teacher or mentor, and use his/her advice to guide you in the selections for your portfolio. Don’t send twelve similar pieces–show the schools that you are multi-dimensional and capable of working in all kinds of mediums, but do show your best work. Make sure when you show the photographs of your work they are well lit, in focus and close-up. Enlist the help of a friend who's good with a camera. Take advantage of portfolio days if the colleges of your choice offers them. You can drag your actual work to a "fair" and have it critiqued by faculty members from the schools of your choice. Sometimes, students get admitted on the spot!
Music portfolios, CDs or tapes should be recorded in a studio, if possible. If you are going to the colleges for auditions, make sure you create some kind of grid to keep you on top of these appointments. There are all kinds of requirements (like listing what pieces you will play, which you have played, who you’ve studied with, what books you’ve used, etc.) Follow the directions and relax–you’ve probably been doing this all your life, and it’s what you love most in the world. You can do it with ease.
In both situations, make sure you know the school's deadlines and requirements for portfolios, auditions, tapes, etc. This is your job - stay on top of it.