I have only done sprints and olympic distance, may differ the further you go.

I swim in jammers (swimsuit that look like bike shorts but without the pad, $14-$20 at your local sporting goods store) with a wetsuit over them.

At T1, I take off the wetsuit and put on a jersey, socks, bike shoes, sunglasses, helmet.

At T2, I take off the helmet, bike shoes and put on running shoes.

If and when you really get into it you can:
Leave your bike shoes on the bike and do flying mounts and dismounts,
Buy a one piece tri-suit that combines the jammers with a sleeveless jersey
Go without socks and/or sunglasses