I Wish i...

1. I wish I knew this before
2. I wish I had learned english in the school (it's better not to use the article here)
3. I wish I would go to Japan

I Wish I...

1. I wish I knew this before (You can also say, "I wish I had [I'd] known this before", but what you have is perfectly correct!). 
2. I wish I had learned English in the school.
3. I wish I would go to Japan. (You would use "would" if it is something you can do, but just haven't done.  If it's something you want to do, but can't, you would say "could")

I hope you will get to visit Japan soon! Good luck in your English studies, Aleksandr!