The bare minimum
- a bike ( mountain, road, time trial, bmx whatever ;) )
- tri suit (something you can swim in and cycle)
- running shoes
- a race belt if you don't want to have to try and put on a jersey (with your number attached) when your wet!
If it's an open water race you will probably need a wetsuit.
Of course there's loads of other stuff you can buy.
"Must tri harder"