I am now about to turn 17 and will be finally beginning the application process to seminary(ies). I first had thoughts about the priesthood at about the same time as you, right after I turned 15. From personal experience, I would like to lend a few tips about the next year and a half or so in this thing called, "discernment."
When I started, I didn't really know what discernment was. I decided that to really kick my vocation into gear I just HAD to pray 24/7 and be the perfect Catholic in all ways. I realize now I put myself under a little too much pressure which only hurt. I became a little too devoted and this caused me to easily get tired and lose faith. Only in the last few months have I understood that discernment is a much more casual and open process than you may want it to be. It would be nice if there were a website a man could find and just go down to checklist to become a priest. Unfortunately, no such website exists--believe me, I tried to find it.
Instead, I would recommend that you simply (remember that simple isn't necessarily easy) do as Jesus commands in the Gospel--increase in virtue and decrease in sin and vice. Life is a long, slow grind toward the end, and expecting to leap up right NOW and become sainted and sinless is unrealistic. Instead, take each day one at a time and contemplate it, asking yourself questions like:
--What sins or bad actions have I committed and why did I commit them?
--Have I grown in virtue and understanding today?
--Have I helped someone weak or in need today (remember that God is visible in the faces of the downtrodden, and try to broaden your definition of downtrodden. The overweight, acne-covered, annoying "loser" in your class could be just as downtrodden as the panhandlers downtown, but the guy in your class is within your reach to help. God presents everyone with opportunities for mercy, we just have to reach out and take them by the hand.)
And so on.
At the same time, allow prayer and the sacraments to grow in importance in your life. Don't be unrealistic--2 hours of prayer and scripture reading a day will just make you more likely to skip it all together the next day. Even just a few minutes of prayer or a few verses of the Gospels can give you enough spiritual nourishment to last a long time. If you have read or watched The Lord of the Rings, remember the lembas bread--just a few mouthfuls could keep a man full for a day. God's nourishment is powerful if we recognize and appreciate it.
On the issue of sins, do not despair! I am still struggling with sexual sins now and I suspect I will continue to for years to come. But don't let that get you down. Loss of hope is the devil's triumph; Steady perseverance are his enemies. Your greatest weapon is the confessional. No matter how many times I go to confession, the sensation of spiritual relief and happiness is always great beyond my expectations. Don't be afraid to ask your parents to take you to mass early so you can go to confession, and don't be nervous about asking your pastor to hear your confession. Your parents may be curious, but they aren't going to pry about what you are confessing, and I can almost guarantee that your pastor would be delighted to have a teenager going to confession regularly. You will not rid yourself of vice until you enter Paradise, but God has given us miniature paradises on Earth in the form of his sacraments and the spiritual life. Use them, and soon the positive power of virtue will overcome the negative power of vice in your life.
Above all, remember that there are people out there struggling with their vocation just like you. Use these forums, talk privately to your pastor or another priest, or a youth director or relative or anyone you trust. It is too early to be meeting with a vocation director at this stage, but there are plenty of people and resources to help you.
God bless you, brother. Remember to pray for everyone discerning their vocation, and don't become bitter, angry, withdrawn, or scared. God is with you through his Spirit, and the world is full of good people. Even if things don't turn out as you expect, God still loves you and will welcome you back every time you fall.
Thanks for reading this rather long post,
P.S. There's no "required reading," but C.S. Lewis and Matthew Kelley have some great inspirational books and talks--especially impactful for me was The Screwtape Letters.