Introducing a Newbie

Welcome to BT!  Here are my thoughts on your questions.

For clothing, I keep it simple.  Tri shorts and tri tank for the entire race (I'll break that rule if I ever do an Ironman and will probably have a back up tri tank and shorts in my gear bag in case I'm uncomfortable at my half).  Some people wear one piece racing suits.  You'll also see some people pull shorts and a shirt on over their swim suit.  If you go that route, make sure to practice to be sure you don't have chaffing issues.  If you do have some minor chaffing, you can apply body glide or Aquaphor to the affected area.  I had some chaffing issues on the bike and now apply Aquaphor before every ride longer than an hour.  I used to use Chamois Butt'r but it seems to wear off after about an hour.

For the transition area, I keep it simple like this (that isn't me, but I think the tri clothing sizing chart was set up for people that thin :-)).  Just like the video, I fold a towel in half length wise and put it next to my bike.  I then put things down in the order I will use them.  Bike shoes first with my socks in them.  I put on the socks before the race then roll them off and put them on top of my bike shoes so it's easy to roll them on to wet feet quickly.  My helmet goes upside down on my handle bars  and my sunglasses go upside down inside them.  I put them on a couple times to make sure they are lined up.  I put my running shoes and race belt behind my bike shoes and my bag of gear behind that.  I keep a bottle of water and a dish towel near my gear bag in case I need to rinse my feet.  I don't eat or drink in transition.  I wait until I'm on the bike and settled down.

Make a list.  BT has a handy list here.  If you are like me, you'll be double checking everything several times.

Once I'm set up, I count the rows from the T1 entrance to my bike and the T2 entrance to my bike.  I then walk over to the lake or pool, and jog really slowly toward T1 visualizing myself taking my swim cap, goggles and starting to take my wet suit off during the race.  Jog to my row, visualize stripping the rest of my wet suit, sock 1/shoe 1, sock 2/shoe 2, glasses, helmet and jogging toward the exit.  I repeat the visualization for T2.  You'll see all kinds of crazy stuff during the race, just concentrate on having a smooth transition and breathing to relax.

The biggest thing is to make sure you have fun!