I can't speak to how to fit a tt bike - I had mine professionally fit... but I can talk about starting.

Most TT's you start out being held by a race official. You won't get the fancy starting ramp you see in the Pro races, but someone will stand behind you holding your saddle while you are completely clipped into the bike. This is not scarey! I promise they won't drop you and you won't fall over when let go! If you feel like you aren't being held up straight do ask the person holding to move you - they will be happy to do it. You want to start out with your less dominant leg a bit above the 3o'clock position (this is because you want your stronger leg to be the one pulling over the top for the best starting acceleration). If you have aerobars DON'T try to start out in them. Start out either in your drops or out on your cowhorns if you are set up that way. There will be a second official giving you a count - they'll ususally position you at about 30 seconds to go, and the official will give you the last 5 seconds count out loud and with his hand. When they say go, you will be let go and you'll want to get moving. Most people once they are moving (a few pedal strokes) will stand and get up to speed, then get into the aero bars.

For a lot of people the idea of being held sounds frightening, but remember these officials hold up big guys who are a lot heavier than you - they won't drop you. If you can find someone to practice with a few times it helps a lot , not only to alleviate any fears, but to get the feel of how to start off nice and strong too.