Rocknrollgirl's advice is really great, especially if you plan to self coach.

I'm not a tri person, but I figure motivation and planning training in the general sense isn't much different for any sport.
So I started racing last year. I did my first season without having much of a formal plan, really just to see if I could do it and could be competitive. When it came time to think about the next year I knew that if I wanted to improve and move up I'd need to train with a lot more focus than just go out and ride most days...... I figured sure, I could read some books and come up with a plan for myself, but it would be better (for me) to just go to someone who already knows what they are doing, so I hired a coach. It also gives me someone to be accountable to. It's been working out quite well for me. I hired someone who isn't totally invasive - doesn't monitor every bite of food I take, etc., but gives me guidance and workouts specific to my upcoming races.
There are a lot of levels of coaching. My coach uses Training Peaks, but puts in my workouts herself. You can sign up for a computerized training plan there fairly inexpensively (as little as $10 a month and follows the Friel model - which is periodization).
My coaching is fairly low level, I get workouts and consult personally a few times a year (of course she's a team mate so I get to talk informally a lot) and not terribly expensive -$75 a month
I have team mates who have a pretty intense coach, who gives a lot of hands on stuff - they do rides together, get nutrition information and a lot of testing - he charges around $250 a month!
Of course you can find a lot inbetween too.