Agreed - start with a pool based sprint. If its local go and swim in that pool and have a ride round the course too. In training, practise your transitions - i do this by short fast bike/run/bike /run sessions (eg 10mins bike, 1km run, 10mins bike, 1 km run etc) - gets you used to getting your bike gear off and your run stuff on in a hurry while out of breath! Clubs can often organise swim/bike brick sessions too - great for practising swim to bike transition, which is a tricky one unless you know the pool owners/organisers. Enjoy and good luck!