First and most important thing is to figure out what exactly you want. Are you planning to to 4 way belly, becoming an aff instructor, freefly, tunnel coach? If you show up without knowing what you want, you're not going to get as much out of it (it will still be fun as he'll though).
Regardless of what you want to progress in, get a coach. Theyvare invaluable.
If it's your first time in the tunnel, I would recommend not doing a lot of time within a short period of time, especially if you plan on freeflying. I did 50 minutes my first time in 10 minute blocks with two minute intervals over two days and couldn't walk or do much else for a week. I'm in pretty good shape and work out often. Second trip I thought I would have better tunnel fitness and booked 70 min over two days. Uhh, couldn't walk for a week after that too. After about 4 hours total Its getting a bit easier. I did 90 minutes last week and am recovering a lot quicker.
I loooove the tunnel