Coach or training site?

I'd suggest looking for a local club wher eyou can learn from experienced members. You might be able to afford a coach, prices range typically as low as $50 / month to as much as you want to spend. I think an average price is $150/month perhaps?

Before you put down money for a coach, I think ou shoud pick up some basic books (going long, gordo byrn is good, there are many others) and talk to local folks.

There are plans here too.

Of course I'm not addressing the question of doing an IM in a year at age 18. Certainly can be done, but if you are viewing tri as a long term adventure into your 20s, 30s, or longer, consider getting really good at Olympics this year and choosing one sport to "go long" like training specifically for a half marathon or marathon or a century ride or similar. Endurance athletes peak in their 30s & later. Young folks have a gift of speed and it'd be a shame to not make the most of it in my opinion.