[chryuop]

You start realizing what you learn and what is used when you read Japanese books. I am in that stage now. It is terrible when you see the "grammar" you learn has several changes and "words" assume a different look. You will start getting no longer confident in what you know and what you have to use.
I write to my penpal and wonder if they find me too "formal" and they reply as formal coz I write that way. But on the other hand you get afraid of offending if you get too "informal".

I think there is not a safe recipe and talking to native speakers and reading will slowly get us used in recognizing the proper forms to use.
Anyway, if I had a foreign friend who tries to talk to me in my language (Italian), I wouldn't worry if he makes mistakes, sounds like a grammar book or speaks too formal. With time I would help him to learn.
So I wouldn't worry too much in the beginning about the way you sound (at least, unless you are offensive lol). Getting your concept through to the listener and understanding what they want to communicate to you is the primary target now.