I just started Japanese myself, but I speak Chinese almost fluently....
One of the biggest problems in language learning that I've seen, is
people not learning daily life words first. It might sound like common
sence, but its not, 'cause few people do it. When a child learns his/her
mother tongue, they don't learn from a text book like we do.
First: they are completely submerged in the language, even if you don't
speak any Japanese(which I of course know is not so), and even if you
are not living in a Japanese environment, you must create a Japanese
environment. One where you don't allow yourself to use English unless
absolutely necessary. (I have a friend who learn English with this alone)
Second:
Learn First things that you will use every day, and when you learn them,
use them every day, maybe even replacing there English equivalent when
you are talking. A child learns language from trial and error, don't be to
afraid to try to use what you know. After all, failure is the author of success.
What my friend that I mentioned earlier did, was she made some English speaking
friends, and when with them, they made a rule to never use Chinese unless they
didn't know a word; in which case they would look it up and say what they wanted
to say again, this time, of course in English.
Third:
Try to 'think' in Japanese, like when you count in your head, try to use
Japanese numbers instead of English ones. Replace in your mind, English
words with Japanese ones.