1. Decide right here and right now that you're going to be yourself, no matter who has a problem with it. Sad but true, people will take advantage of the fact that you're young and trying to find your place in the world. Specifically, they'll tell you that you're idealistic and try to squeeze you into a mold that fits their purpose for life, but holds absolutely no benefit for you. Stand firm, and be yourself. When the end comes and we stand in judgement, we won't be asked why we weren't this or that to win the approval of this person or that person. We'll be asked why we weren't ourself. Avoid the question, and decide to be yourself, ideals and all, right now, and forever more.
2. Learn discernment. Or rather, sharpen your intuition. It's telling you things for a reason, and you'd be well advised to heed it. The good, old fashioned common sense that will help you to learn from your experiences is what we older folks refer to as "the school of hard knocks," because this is the kind of skills and knowledge that you don't find in books or classrooms. You aren't done learning - rather, you've evolved to a new kind of learning. Now you learn from experience - both your own, and other peoples'. Mine it for all it's worth. There's no substitute for being able to assess a situation quickly and move based on being able to trust your gut - and the older you get, the more important this is.
3. Pick your battles strategically. You could spend your whole life in battle. People are always going to challenge you. That's part of being an adult with a mind and a will of your own - people will try to sway it to their own good instead of what's best.