Ok so there's a lot of things you could do your first day on the job in a youth ministry context. Depending on when you start your first day may be a camp, retreat, missions trip, event or something else. I'm going to write this blog as tips for your first day assuming that day is happening at the church in your office space.
Several years ago my first day on the job at Bel Air Presbyterian Church involved me opening up the door to an incredibly messy and slightly disgusting office space. I remember walking into my office and seeing two desks and a bunch of camp looking stuff laying in piles. I sat in my chair, leaned back and then started to rummage through the desk. The first thing I found when I opened middle drawer was a half eaten bowl of cereal still with milk in it and a spoon. (no I didn't eat it) After a few minutes in my office basking in the excitement of having a great full-time youth ministry job I decided to go on a walk around the church.
My recommendations for the first day on the job is to spend a lot of time connecting with people and do everything you can to remember some of the names you are meeting. Remember that you are stepping into a new system and you probably don't know how things work there very effectively. Ask a lot of questions. Get out to lunch with some people. Sit down and ask straightforward questions about landmines you want to avoid, people who you need to be aware of, systems that might be different from anything you've ever been around before.
The first day will go a long ways to establishing in peoples minds who you are. It's like having a first impression with a whole bunch of people all at the same time. I typically try to get someone to break the ice for me and walk me around and introduce me to people. You might be the type to be able to go around and do that on your own but having someone with you can give you some info about them that would be helpful to know.
The next thing I do on the first day in a job is to just try to start getting the lay of the youth ministry and what are things that you need to be aware of. Let's just hope you have an administrative assistant or even better other youth ministry staff. Meet with them and find out what is going on that day, week, month. Start figuring out the "rhythm" of the structures currently in place.
Two more things are super important that first day. After getting a good dose of the place you will be working and some of the people you will be working with I recommend finding a place to pray. It's easy to get overwhelmed on the first day in a job. Remember that you will only be a great leader if you are following what Jesus wants you to do. I pray for discernment, for courage, for understanding, patience and for ears to hear everything being said.
Lastly remember that in your first day on the job all that wasn't talked about during the interview process will probably be made known to you. Chances are good that most of the staff you are spending time with today weren't a part of the interview process. They probably have a ton of things to say about the church and the youth ministry and most likely will tell you all about a bunch of stuff that you didn't think you needed to know about. I recommend listening to everything they say but filtering it through the lens of what the search committee told you. It might be that the search committee recognizes that there are a lot of things that need changed in the ministry and you were brought in to change them. The people in the office you talk to may or may not be aware of those conversations or expectations.
Have fun on the first day. It will only go uphill or downhill from there. 
