Don't buy expensive brushes as a newbie - buy them when you become more experienced, and want to really invest in some good tools
Don't overdo it - less is more
blue eyeshadow is a NONO unless you can really pull it off/apply it well.
Look on YouTube for amazing tutorials - juicystar07, allthatglitters21, xsparkage, and whatstyleistonickel are all amazing youtubers
BLEND - into the hairline, down your neck, everywhere! Nobody likes to see lines or splotchiness.
Ask for help - from your mom, mac counter ladies, friends, anyone!
Touch up mid-day - when yOu're in school, just put powder to mattify your makeup, it makes a WORLD of a difference
don't put eyeliner on your lower waterline - it just looks too harsh
HAVE FUN!