where to start?

hey everyone i was wondering if someone could give me some good training tips for doing triathlons, i plan on entering a sprint tri for my first event, it's a 500m swim, 20k bike and 5k run, i need to work on my swimming the most as its my weakest could someone help me write up a programme, i haven't got round to doing swimming lately but i did do the bike and run, i managed 28mins for the bike and 18mins on the run i haven't got a clue if these are good times or not for sprint events?