Tri newbie here - do I actually need a swimsuit and a good bike?

I’ve always wanted to do an Ironman and I enjoy running and biking on a cheap gravel bike from walmart (ozark trail) and I love swimming although it’s been a while (I was a lifeguard 10+ years ago).
I want to force myself into doing a sprint just to get the feel of the race before but any better gear. Can I just swim in a bathing suit and swim shirt and can I use my old gravel bike or will I just look like an idiot when I pull up?