Get it booked and get over there. Theres a good few campsites on the island and not finding somewhere to stay is not a fear. Proper bricks and mortar digs will cost a fortune though and its way too late to get them as they will be booked already. Physical building accomodation is tight in tt weeks. Been going to for twenty years (not to watch races, more for the riding and atmosphere). Riding flat out over the mountain is a hell of an experience. This year the max is going to the island for the first time, looking forward to that a lot, always been on Ducatis and 750 jap sports bikes til now.
Dont risk speeding in areas where the posted speed is there. Fines are heavy. Derestricted signs are just that, derestricted speed, but dont do anything stupid (150mph isnt stupid in the right places). Overtakes as long as safe are fine at any speed, but go wide, cross a double white line, thats stupid and will get you a dangerous driving ticket, a huge fine, a ban for the duration of the tt and potentially jail time.
Dont ride near the centre of the road if you are not riding fast, people who are riding fast will get irritated and pass you as soon as they realise you are easy pickings, some idiots will actually pass you on the inside if you are in the way for too long.
Apart from that, have fun. Ride fast, have fun, ride faster, have fun.