I'm probably going to break this up into a few different parts, so skip any parts that don't really matter to/relate to/interest you.
There are lots of different routes to take to become proficient in Japanese. You CAN probably try to learn it on your own with books and stuff, but the best method by far is to take Japanese classes.
The sooner you start studying, the better. If you're in high school and your HS offers Japanese classes, take them! And take them seriously, even if no one else in your class/school does.
If your high school doesn't offer Japanese classes, but you're in high school, you might want to check nearby community colleges or universities. They'll often let high school students take classes during summer session. That's what I did, and I actually managed to get 2 academic years' worth of Japanese classes under my belt before REALLY entering college. And since I did those over the summer, I learned about 2 years worth of stuff in 3 or 4 months. Intense, but effective if you're genuinely wanting to learn.
When choosing a college or university after high school, be sure to look around and do your research. Some schools only offer 2 years of Japanese courses. Some offer courses that go up past the grad student level.
Obviously, take as many Japanese classes as you can when you're in college. Any related courses, like Japanese history, art, cinema, calligraphy, culture, etc. are all very good to take too. They might be kind of boring (okay, maybe VERY boring) but I often thank the stars I took these classes, as sometimes you unexpectedly need to have historical knowledge or other knowledge when translating, or even when just interacting with people.
When studying Japanese in college, there are a couple things to realize, and I only learned them and realized them after my many years.
I recommend studying abroad in Japan around your third year of Japanese. Meaning you probably took about 4-6 semesters of Japanese language classes previously. The reason for not going earlier is because you can learn the basics of the language anywhere at all. But the more intricate stuff can't really be taught, it needs to be experienced directly. So around the third year is a good time for this. Of course, if you happen to be really rich or something, then sure, go as often and whenever you like!
Now I know many people reading this will be like, "But I can't afford that! That's too expensive!" That's what I thought way back when too. But man am I glad I didn't listen to myself. Here are just a few things about this I can think up real quick:
So if the idea of studying abroad seems crazy to you, give it some more thought. Incidentally, I recommend the Tokyo International University Japan Studies Program -- I spent a full year there many years ago and came away with a pretty awesome experience overall. (Just look out for the people I mentioned often show up in first-year Japanese courses) The teachers there are excellent, and staying with host families taught me so much that I wouldn't have learned about had I stayed in a smelly dorm. I also seem to have left a legacy, so be sure to mention me to teachers and other people if you ever go :P I'm somehow famous (infamous?) there now. Mentioning "pink buses" might jog memories too! Don't forget!
After you've had tons of awesome experiences in Japan, it'll be time to come back and finish up your degree eventually. You can probably have some credits transferred from your school in Japan, so that should probably take care of most of your Japanese courses and you'll probably mainly have crappy unrelated stuff to do. But do them and graduate.
And of course, alllllll the while during all of this, always always be studying hard on your own!
By the time you've done all this, you'll probably be very proficient at Japanese!
But if you're aiming to be a pro translator, you also gotta learn how to translate!
(to be continued, been too busy lately to finish this)