Here's my humble opinion: (See following articles for detailed explanations and tips for each sub-section)
BUILD TECHNIQUE: This is the training camp of music. Just like an athlete will get stronger just from participation, your technique will get better over time from the experience of learning songs. However, if you can devote a few minutes each day to focus on technique, your skills will grow much quicker. Just those few minutes will help improve your skills, so you can more readily enjoy playing music later! Serious athletes train (run, lift weights, stretch) so that when it's time to perform, they are ready. So should serious musicians. As with most skills, the more time you devote, the more rapidly you'll progress. A daily routine with a significant focus on technique will result in dramatic growth within a period of months.
ATTACK THE ASSIGNMENT: When most people think of practice, this is what they think of. They play through their assigned pieces a few times; maybe slowly, maybe fast. Sometimes they need to play an assignment a few times, and on the third try, they get it right, so they move on to the next song. So now they've practiced it right once, and wrong two or three times...which way are they more likely to play it again under pressure?
PLAY FOR FUN: Isn't this why you play an instrument? It's fun! Always end your practice by playing something you like to play. It may be what you just learned when you attacked your assignment, or a solo from 2 years ago that you memorized for a school concert. Whatever it is, play for fun for AT LEAST AS LONG as you Build Technique.
YOU'LL GET RESULTS! I began playing in 5th grade. Starting in 10th grade, I began a routine similar to the one above. I went from being about the 3rd or 4th best sax payer in my school to one of the top 10 in the state of Pennsylvania by the beginning my senior year. Though I wasn't a music major in college, this routine allowed me to become the lead tenor player (beating out other music majors) in the jazz band. As a private teacher, and professional musician, I still have found that when I apply this routine rigorously, my growth as a musician is noticeable. I urge you to adopt this routine. It will build discipline, confidence, and allow you to more fully enjoy music.