I think first season can be anytime between 6 months and 18 months or so.
My pup was 14 months.
You'll hear all sorts of conflicting advice on when the best time to spay is, and there's pros and cons of each. There's a good article somewhere about it. I'll see if I can find a link to it.
Generally speaking, if you decide to let your pup have a season first before spaying (that's what I'm doing), then the best time to spay is mid-way between first and second, i.e. 3 months after the first. I believe the health benefits of spaying are reduced if you wait until after the 2nd, 3rd, 4th, season etc.