Couch to podium in 100 days

Take it slow and work your way up. It's easy to get injured and/or burnt out when you go out of the gate with both guns blazing. Endurance sports in general are about consistency.

There's plenty of great advice and plans (even some free ones) on the site.

Have fun. That's what it's all about in the end.

I'd second this, but more for the reason to get your feet wet (literally and figuratively if its a tri) without what can be like chaos of larger races like Hy-Vee where there's 2500 competitors. Smaller races will give you more people to compete with and a little more relaxed environment to figure out the race.

Time to lose 15 lbs if you have your heart set on podiums.

Make goals for your own speed and times, regardless of whoever shows up.

I got on the podium in my first tri, but it was a small event (about 70 people and only 4 in my AG). My next tri, I'll be really really surprised if I even make it into the top 1/2 of my age group based on last year's time.

Depends on build. My boyfriend is 5'11" and would look skeletal and unhealthy at 170, forget 155. 

Sorry, I didn't mean to imply it would look good but 170 at 5'11" is big for this sport if you want to go as fast as possible.  At 6'1" and 170 I've still got a fair 10lbs I can shed before my A race this year.  

Don't underestimate the difference between being "comfortable in the water" and swimming a race in open water.  Far different than even swimming a race in a pool.  People tend to panic; just wait a month or two until we get some good OWS weather and the threads will start to pop up.

I recommend getting in some open water swimming multiple times before your race.  I also recommend you do it with other people.  If you're really motivated you can simulate the swim start like this: http://www.youtube.com/watch?v=r3S0wu4Zbfk

I hate to say it, but one useful pointer would be to just read around these forums for a while. They're full of lots of useful input. To reach your potential in tris you need to be a student of the sport, and learn not only about the differing needs of the three sports, but also how to smartly balance them in your training. Lots to learn, and that's part of the fun, and BT is a goldmine of information on that. But coming onto the forum and saying "I'm new and very competitive, please give me some pointers" conveys a reluctance to do your homework.

Take your time, enjoy the journey, and try to specific things that you should work on, based on your current fitness and experience. And hang out here at BT - it's a great group of people.

Having said this, if you're only 28 then you're probably less injury prone than many of us older farts, and you also might well get a high AG placing in your first sprint. There aren't so many guys in their 20s in most local races - it's ages 35-50 that tend to see the largest numbers.

The answer to getting faster on the bike is almost always "lose weight".

IMO, the key to being successful is injury avoidance.  That is what intercedes most often to not allow consistent training.  And consistency is the key.