Today I had my interview for a job that I had no idea about (my uncle told me to meet XYZ person, and my uncle himself didn't know anything about the position available).  

So I went to the office of XYZ and met with him and my interview was quite a good one and he said that the job was mine, if I wanted.  

(BTW. XYZ is the CEO of the company).

The job is a type of managerial position and I will be responsible for one sole department. The person who is currently managing the department is not responsible enough, so although he is quite senior to me (professionally and age-wise) he will be under my authority and will report to me.  

I will have to manage all the employees of the department, making sure they arrive on time and do their work properly. I will also be responsible for making sure the client's complaints are resolved in a timely manner.  

I will also have to manage other couple of things.

This all sounds good, but the problem is that this is going to be my first proper job (I have done odd jobs before). I am a 22 years old male and I have just completed my bachelors.  

I have no real experience in the market or of having managerial position.  

The department employees have an average age of 30 years so I don't think they will like to take orders from someone of my age.  

Also, XYZ told me that I can't be their friend so I will have to keep my distance to maintain my position and respect.  

I think the previous head of the department will obviously dislike me as I am kind-of replacing him. Also, other employees may generally dislike me due to my age and position.

I need some guidelines on how to handle this position. I am thinking of giving a formal introduction of myself and explaining to them that what I am here to do, what are my responsibilities and what I expect from them.  

Is this the best way to go or should I just quietly sit at my desk on the first day and meet employees one-by-one?