SKIN: First things first make sure you wash your face and make sure you don't have any previous make-up on your face, Now you some concealer to cover up and acne you may have i recommend Maybelline's Concealer Cover Stick Corrector.
Now add some foundation, this look has quite alot of make-up but it doesn't look overboad, so just simply use foundation to go with your skin tone, i reccomend Maybelline's Dream Matte Mousse Foundation as it's easy to use and spread.
Demi doesn't really seem to have alot of blush on but you can use a cheery colour if you wish.
EYES: Now the eyes are important, you need to find a blacky bluey colour more like a navy, but i recommend using to colouyrs and blending make sure you don't go above your eyelid! I recommened using these two colours: Maybelline's Expert wear eye shadow's singles- green/blue and Maybelline's Expert wear eye shadow's singles- black
Now you need some eyeliner, if you look at demi's eyeliner it's alot thicker on the outsite of the eye and thins out working towards the inner part of the under eye, so work out from the outside of the eye into the inside, remebering do not do it too thick but i'd recommend doing a thin line and another thin line underneath that line =) No particalar eyeliner but i recommend: Maybelline's Define a Line
Now Add mascara lightly, if you look at demi's lashes there not long, but you can see mascara is visible there. Just black masacra but i recommend: Maybelline's Lash Stylist Washable Mascara.
LIPS: Demi doesn't really have any visible lipgloss or lipstick on, but i would add a lip balm.