First I just want to put this out there: Be prepared. There's a really good chance once you've done your first you'll want to keep on doing them, and doing them, and doing them again. Triathlons, for whatever reason, are highly addictive.

But let's assume you're able to fight the cravings and just do this one:
1) Safety first. Take your bike to a local bike shop and let them inspect it for safety. Make sure you have good quality running shoes, a bike helmet, and I HIGHLY recommend buying a RoadID. Nothing is more important than YOUR safety. Don't dismiss any efforts to make your training and racing just that much more safe.

2) Fun comes next. Triathlons are such amazing events. (But then again, I'm addicted.) Remember during your training, and especially on race day, to have fun. Enjoy your ride. Crush that run. Rip through that swim. And savor every action packed moment of it. Nothing makes for a better race day than a fun race day. Find fun places to train. A park with lots of wildlife and scenery makes a great place to run or bike. A local park where friends hang out. A peaceful stretch of rarely driven road. A trail along a lake. Just wherever you find fun, train there.

3) Gear: I wear triathlon shorts on race day, plus a shirt that I put on after the swim. You don't need these. I know many women race in their swimsuits. But I can't advise on this as I have no experience whatsoever. I do love my tri shorts though, as I also run races in them all the time. Bike helmet and good shoes (see #1), goggles for the swim. All essential gear. And of course a trusty bike.

4) Training: Make sure you can bike the entire distance before race day comes along. If you train on your trusty bike but never train up to 25K, there's a chance on race day that you wear out early and struggle on the run. This is NOT fun, and goes against #2. Make sure in your training that 25K on the bike may be tough, but doable. The same with the run distance and the swim distance. Triathlon isn't like say, half marathoning, where if you can train up to 10 miles, you can race 13.1. In triathlon, you want to be able to do ALL the distances, with minimum struggle.

Also, there's a type of triathlon specific workout called the brick workout. That's when you bicycle for a while (say, 30 minutes) then get off your bike and immediately run for a while (say, 30 minutes). A problem in triathlon is going from cycling to running. It totally messes with your body - in particular, your legs. So doing a workout to get used to this wacky phenomenon is quite handy dandy. If you throw some bricks into your workout schedule, you'll be glad you did.

I don't know what to add, except I'm really excited for you! Triathlons are so much fun. I wish you happy training and joyous racing. And I think it's really cool that you're doing this for a cause. Very admirable, indeed.

I hope this helps.
TriJoe.

Quit Calling it a "mini-triathlon"
*train hard
*have fun

I've done about 6 try-a-tri's over the last few years and my first sprint last summer. If you are swimming indoors, you will be fine in a swimsuit. I have my shorts waiting for me at transition and you can also throw a t-shirt on over top as well - your transition time may not be great but it's a good chance to catch your breath if needed and put on the clothes over top that you want. One piece of advice - I wear a sports bra under my swimsuit - my girls are not comfortable without the extra support!!! Secondly, try a workout or two in your workout gear of choice - better to try out what you want to wear and make sure it is comfortable and won't chafe BEFORE race day. Being prepared is key to success. Good luck!