Just some basics here. The island itself is really crowded, I mean really crowded, during TT week in June. Island population doubles with bikers. Great for the festival atmosphere, less so for actually riding around. Police are amazingly tolerant considering some of the lunatics riding their bikes, but then I think the IOM people are great anyway. I recommend not riding the route on Mad Sunday, as too many loons around. You can do it on a quieter day in the week.
So if you want the full festival, go to the TT. If you like things a little quieter, consider the Senior week later in the year. Less high profile, less crowded, and a more relaxed atmosphere.
For the TT, book your accommodation well ahead, I mean this year, as anything decent goes quickly. With a bike, easiest route is by fast ferry (a seacat) from the UK (Liverpool). About 4 hours if my memory serves me well (which it often doesn't). However I think the seacats cannot always run if sea conditions are really bad, in which case you could be rerouted onto a normal ferry which takes twice as long. This is just a risk you take.
Flying? Not with a bike as too dear. Even without, book well ahead. In any case you should book well ahead even with the ferries if you're taking transport. It's the only route where I've seen car drivers as a tiny minority - just thousands of bikes.
Douglas is the capital, and the best place to be if you want to be at the centre of everything. But I've also stayed in farmhouse accommodation further north on the island and had a great time. The course runs all round the centre and north of the island, and with over 37 miles there's plenty of places to view from (though all the best corners fill up quickly). And plenty of restos, cafes and pubs all over the island to refresh yourself.
If you have any experience in marshalling for road races, you could be interested in marshalling. I've done this for the Senior. Advs - you get a great view in front of everyone else. Disadvs - you have to be alert and know what to do if a rider or group comes off in front of you. Also, no photos allowed by marshals.
Whatever, the fact is that wherever you go on the island you'll be surrounded by bikers. So it's a great atmosphere. Imagine Sturgis but without the Harleys! And the people are really friendly. So with a bit of planning to secure your accommodation you should have a great time.
IOM Tourist Office is also very helpful, and has lists of B&B accommodation.
This is a bit of a brain dump, but hope it helps.