I started work at a lodge when I was 15, I'm 16 now and just ended my second season. March, April is usually too late as they have hired most of the staff. As far as experience is concerned, I had been in Vancouver for 2 years and landed my first salmon about a year before by the time I had the job, so fishing experience is not all that much important as a good work ethic, and be able to back up this ethic. This is not the same if you intend on being a guide though, you need as much fishing experience as possible, but just about no one guides full time at the age of 15, I guided a few guests last year with some results. At your age, you would most likely be a deck-hand, that's what I am and was last year too. It's pretty good, get to go fishing quite a bit in pristine wilderness locations and paid to do it !!!
Also, you need to decide if you want to be at a salt-water resort, or fresh-water. I chose salt because that is where I've fished the most oin the past, and I just like the ocean (not to say one type is better than the other). Stick with something that you are more experienced in, this is because guests will be asking you questions constantly and you need to be able to answer them, and not just BS your way through it.
The most important thing you need is to be is mentally strong as well physically strong, the work you would be doing can be quite draining, but at the same time, living in isolation for a few months can hurt too. I've done this for 2 seasons and have no regrets. It's a great way to spend a summer, especially at your age, make some money, catch some fish, meet some really cool people.
Hope it helps, and tell me what lodge you end up at if get into one.