This is my first season of doing tris too. Congratulations on getting bit by the tri bug. I have done one and they are REALLY fun. I have my next one this coming weekend.

I didn't want to invest a lot initially so I bought tri shorts from TE (I got the TYR brand and really like them. ) TE has the BEST customer service and I got them very quickly.
On the top under the wetsuit I wore a heavy black sports bra. The intent was to pull on my running top, but for the bike I changed the plan and just went as is, then pulled the top on for the run when I was dry - much easier.

My training partner who is also in her first season wore a one piece suit and loved it - I just didn't want to make the investment at this time and probably never will as what I am doing works and I am only doing sprints. Also I like the freedom of not being covered by a lot of wet material

If you do go w/ the one piece the one thing my training partner forgot was how to attach the number as you don't need it for the swim or bike. Luckily someone had an extra race belt that she could borrow.