I agree with the other posters on this thread. You need to have as much exposure to Japanese as is possible. Immersion in a Japanese-speaking environment is ideal, but there are people who work hard at gaining exposure by reading lots of books, magazines, etc., and watching dramas and anime.
There is no quick solution. If you're thinking of accurate translation of manga/anime right away, that's very hard because dialogue is difficult to translate. As it is also true in English (or maybe any other langage), there is a lot of things that are "unsaid" and understood in dialogue--it may be past developments in the story, historical and cultural background, etc., latest trends in society, mass media, pop/youth culture, popular terminology, etc.
If you are just practicing translation, I agree with Elizabeth that you should start with "standard" Japanese used in newspapers and magazines. That is how I started, too (in English in my case). It is also good if you can find a native Japanese speaker for a friend or language exchange partner. That way, you learn how Japanese is spoken and can ask questions right away.
I don't know how much experience you have with Japanese, but if you haven't had any extensive experience of immersion in a Japanese-speaking environment, you are doing quite well with your translation practice. Language study, including translation, never ends because you can never reach perfection (even in your native language--unless you are exceptionally gifted).
Sorry I can't be here to answer your questions all the time.
Also, my apologies for rambling...can't organize my thoughts when it's already past my bedtime.