I've started a new thread for your questions.
General: If you're already dreaming in English, you're well on the way. You're doing some of the right things - but you need to get more active (speaking and listening to live conversation).
Look at these definitions; there's not a lot of overlap:
into: Definition and Much More from Answers.com
onto: Definition and Much More from Answers.com
(I can't really think of any, although this comes close*:
Police are looking into it. / Police are onto it.
The meaning of the first is based on the phrasal verb 'looking into', meaning investigating. The meaning of the second is an informal use of 'onto N' which means something like 'beginning to understand'. Another common expression that uses this sort of "onto" is I think we're onto something. [='I think we've discovered something that's worth pursuing as it will probably lead to further discoveries'].
*PS These two uses aren't very close in meaning, but it's possible to imagine a context that allows them both to be used (though not interchangeably).