Depending on the race - there may be assigned spots on the bike racks - according to your race number, sometimes just grouped by wave and sometimes just open. You typically have space for the bike and a towel laid out next to it with your gear. You want to lay out your gear so that it is organized and easy to get to. You should actually practice this - don't want to be hunting around for things.

It is very tough to put a shirt on over a wet body.
Also best not to fuel up in transition. Better to do that on the bike or on the run.
The other thing is to walk the transition area from swim entry to where your bike is and then to the bike exit. Also from bike entry to your rack position to then the run exit. Trust me it is easy to get lost in transition and lose track of where your rack position is.

Be sure to get there a little early and just watch how everyone else sets up. Everyone has their first tri and someone usually gave them some race day help and advise. I've never had anything stolen or disappear from my bag in a transition area. I'm not saying it might not happen, it's just never happened to me. I always leave cash and cards in the car though. If you're used to wearing socks then don't stop now. I see a lot of people wearing them especially as the tri. distances get longer. I've always worn them as my feet seem to blister easily. It takes me approx. 30 seconds to put them on so for me I've decided it's worth the time.

Instead of a cycling jersey you might look at a tri top that you can swim in. Trying to pull on a jersey when your wet can be rather entertaining for the spectators!

Good luck and have fun. You'll be fine.

Personally, I always bring 2 towels, one to lay all of my stuff out on and one to dry my feet off with after the swim. I always wear socks or I get blisters, but I also get blisters if I put dry socks on wet feet. It only adds about 15 seconds to my transition time.

Also, try to lay everything out in order as you'll use it during the race. I keep all my bike stuff together next to my bike (bike shoes, socks, race belt with number, helmet, sunglasses) and then all of my run stuff (running shoes, hat) together on the other side of the towel. Makes it easier to throw things on without forgetting anything for each leg of the race.

One other thing I always bring now after a couple of races in the rain is a trash bag to cover my stuff if it pours. Mostly to keep my keys, wallet, camera, etc dry.