I have worked as a youth minister, and in various capacities in congregational ministry. Others have mentioned that this is ministry, and you're called to it by God and by your sense of how you can serve God and God's people. That is true.
People in ministry are also called by the people in the congregations they serve. It would be helpful for you to ask your pastor or someone else you respect in your congregation to help you figure out whether your call is genuine.
You know, in the Blues Brothers movie, how Dan Akroyd's character was always saying "We're on a mission from God!" When you hear these words in your heart, or when you hear someone else say them, it's helpful to ask "how do you know?" That's what I mean by figuring out whether your call is genuine. Various churches have a service of installation for ministers and church staff. You may want to read through the promises you'll make, and the promises your congregation will make, if you take up this work. Can you make those promises without reservation?
Now you've got the job. What do you do next? Pray continually for one very important thing.
Your supervisor is probably an ordained minister. Keep your supervisor informed. Ask for advice whenever you need it. "What's up with Joey and Charlie? They always sit in the back goofing around. How can I engage them?"
This may be your first job, so you may need to learn to work. Make it a point to show up on time and pay attention. Keep in mind that not every ordained minister is a tremendously talented organizer or supervisor: you may get vague or conflicting instructions. Ask for clarification. Ask for priorities. You might even ask your supervisor, "please help me learn to work effectively."
Prepare. Always have a schedule for an event. If your event involves scripture or other textual content, study it ahead of time. Consider asking for help putting the words in context.
It's like herding butterflies. A big part of the job is organizing and communicating events. You'll need to talk to the young people in your congregation all the time, AND THEIR PARENTS AND FAMILIES. You need to tell them about the Bible study or laser-tag evening or whatever, and then remind them to show up on time. It's surprisingly hard -- and frustrating, honestly -- to get people to show up. You'll feel, a lot, like the flight attendant right before arrival. "We know you have lots of choices about how to spend Friday evening, and we thank you for spending it with us."
Get to know the parents and families, as well as the kids. If you have evening events, invite the parents to participate in the closing worship/prayer circle at pickup time.
Try to have at least one person-to-person conversation with each person in your ministry. Ask them "what's your story" and open-ended questions. Get to know their joys and burdens.
If your group starts with 13-year-olds, get to know the 10-12 year olds and their parents and church school teachers.
Get to know the person who keeps the lists of church members: maybe the office manager. Work closely and cooperatively with this person; you'll occasionally need a favor, and the best way to get favors is to be respected.The office manager probably knows a lot of families' situations too, so you can ask for butterfly-herding suggestions.
Your group may ask you to pray publicly with them. Fine. But keep in mind that your job is to teach them to pray for one another and the world. You're the coach, not the quarterback (to use an American football metaphor).
At least in the US, teenagers don't have many opportunities to learn to speak with confidence in public. If your program permits, encourage your young people to get involved in public reading of scripture and other things to help them claim the authority of the Gospel for themselves. Coach them on clarity and presence ("speak slowly, don't mumble, stand up straight, speak from your heart.")
Don't take anything personally. Don't panic if there are challenging situations. Easy to say, hard to do. Just remember that your group will soak up your mood.
Keep your hands to yourself. A lot of people claim that "goes without saying." Sorry, it doesn't go without saying. Part of your ministry is to love these people. Beware drifting from agape to eros. (Look those words up; psychologists call it "countertransference" and it's dangerous.)
Pray continually. Give thanks in ALL circumstances.
And have fun. Youth ministry is a great way to serve God and God's people.