but you say the Bible is the word of God, the true book, so tell me what are the criteria that you define if a book/message is true? that's what I asked. and what about the other two questions?
Joe made a good point. Your questions are..hymm...very Muslim style

not that it is wrong, but it shows some presumption about God, message (have you noticed that this is word much more often used by Muslims than Christians?), Books that are typical to Islam and rather strange to Christian mind.
That makes answering you a bit difficult. :-[
Muslims seems to come to faith through Book. We through Jesus. I think you have to first believe in Him before you can accept Bible as "holy". There are no tests. Bible is based on Tradition. It is Jewish Scriptures and writings of first Christians passed through ages until now. That is all.
how do we know if message is true? Whose message? Jesus? i read Sermon on the Mount and it "move my soul" (i do know it sounds kitschy

)
And watch out with term "word of God" when speaking about Books. We also use it but - many of us - in different meaning. We also call Jesus - Word of God. The most typical Christian answer I could give you is: for us Jesus is the message, the Word of God and Truth. Because we felt once called by Him, we also accepted Bible. Probably before that, we thought that it might not be that corrupted as it seems

. But in the end - it is just faith.
2.and also if you say that Jesus was crucified to pay with his blood the sins of the people, then what happened to the people that died before Jesus came? Do they go to hell or heaven?
we see it as a kind of "cosmic" act that penetrates both time (in both directions so to say..) and space. He did it for those who where before Him and came after.
3. and also did the prophets that are mentioned in Bible teach trinity? (i'm refering to the prophets before Jesus was born)
Not exactly.

We believe that God gradually reveled Himself (and Jesus is the final revelation). We think that we can find first signs of it in Old Testament. (Jews obviously disagree here). I sure there was thread about it. Maybe you should look for it?