Hi, and welcome!
My question is, what the person who finds my book is supposed to write in the journal? His name, for example, and the next place he left it, or his impression from the book? I mean what feedback will I possibly get?
That's up to the finder - there's no requirement as to what the person chooses to enter. Some write about how they found the book, some provide their thoughts on the book once they've read it, some do both things - and some simply enter "found it" or "fun idea" or "didn't like it"; you never know what you'll get. (A fair number of finders don't write anything at all, at least not right away; sometimes a journal entry will appear months or years later explaining how they found the book in 2008 and have had it in a box in the attic ever since then, but quite a few books have yet to be heard from at all.)
Even among active BookCrossers, it varies; some of us enjoy writing book reviews with our thoughts and reactions to what we've read. Others don't care for that at all, and simply journal with "found it" and "passed it along". It's whatever you're comfortable with.
And also: if I am not the original book owner, simply have found it, after entering the BCID, will I be able to see all the entries in the journal made before?
Yes, you can see all the journal entries on any book you've found. And for that matter, you can see the journal entries on any book at all; if you spot an interesting book in the "recently caught" sidebar, you can click on it to see where it's been and what kind of JEs it has.
Do check out the Site Watch forum for some examples of wild-released books with interesting journal entries. It might give you some ideas, or at least reassure you as to the different ways people journal books!