Here's what you should do now: cultivate your prayer life. Receive the sacraments frequently. Continue thinking about this, and then still your heart and, in the silence, listen to God. That may seem obvious, but it is the most vital part of discerning a vocation. Second, contact your diocesan director of vocations -- every diocese has one. Meet with him; talk to him. He will have guidance for you. And, trust me: diocesan directors of vocation give enormous attention to everyone who comes to them. It's not their job to recruit you to the priesthood, either. Their job is to help you discern your vocation. Of course, they want priests, but they don't want any priests who aren't called to the priesthood, and so they will help you figure it out.
If you can, and if your vocations director advises it, find a college seminary. I particularly recommend the one I'm nearest to, which is called -- and here I go blowing my identity and location -- the St. John Vianney seminary, attached to the University of St. Thomas in St. Paul, Minnesota. It happens to be the largest (and best!) college seminary in the country, rebounding in the last ten years after long dry decades of bad formation and small classes based on a simple formula of prayer and the cultivation of masculine virtue. Its website is vianney.net/ ; UST's is www.stthomas.edu . Your vocations director may have other ideas -- assuming you are even still discerning when you get to college age, which maybe you won't be.
Now, I'm saying you should enter a seminary, and you're probably thinking, "Whoa, Nellie! I'm not even close to that, yet! Didn't you hear me? I'm leaning against this vocation!" And that's because most people think that minor seminaries are all about taking in college freshmen and outputting priests. That's not at all the case. Like the vocations director, the role of a college seminary is to help young, college-aged men discern their vocations while cultivating their virtues and their love of God, in community with other men. SJV, which, as I said, is the best in the country, sees most of its students "discern out" eventually -- they lose half in the first year, half of the remaining in the next three years, and a quarter of the remainder then drop out of the major seminary. And SJV is happy about this. A year or three in the seminary hurts no one -- everyone who comes out of there is a better man for it, and whether they become better priests or better dads (or better historians!) doesn't really matter as long as they're doing what God wills for them. Many of my good friends are ex-seminarians, and they are all holier than they were when they went in (even the business majors). So, if you think there's even a 10% that you could be called to the priesthood, get in there.
The time to discern whether you are going to be a diocescan priest or some other sort of priest has not come yet, so don't start to worry about it... yet.
Purity is a challenge which every man faces around your age. Keep struggling. Try to excise it from your life, as you have been, because impurity separates you from God and makes it harder for you to hear His voice (because you are listening to your own pleasures instead of Him). But, if you aren't able to win that battle right now, don't let your discouragement at your failure discourage you from following your true vocation, whatever it may be. Doing God's will for you, finding your true vocation, can only help you defeat masturbation -- especially in these dark days of easy pornography and the insane common doctrine that masturbation is something we should be celebrating. It's something even many seminarians struggle with when they are just starting out. Hopefully, their time in the seminary makes it easier for them to turn away from their passions and towards God.
I really hope that helps. It's not all the advice in the world, but... you'll find out a lot of things for yourself when you stumble along the path of vocation. And remember: we're all stumbling around in the dark, just like you. So chin up and look forward to whatever future God has planned for you.