Last year I invested in a pair of Tri shorts and a tri top. The "pad" in the shorts is smaller than your typical bike short and I believe is a gel pad so you can swim with it.

My best advice is to practice in what you plan to wear to make sure it is comfortable. I was going to wear my tri shorts with my bathing suit and a sports bra under my suit, but the bra created too much drag so I ended up getting a tri top at the last minute.

Good Luck on your first Tri!