please,i want someone to explain where to start in oracle. Because, i'm novice on it. Should i start from oracle 9g or 10g or sql. And which book is good for a beginner like me to learn oracle database.
Lao, The first thing to do is learn SQL. You will need a database to practice on. Since your goal is to learn Oracle too, Oracle is a good database to start with. I would say to use Oracle 10g rather than 9i since it is a more recent version. It doesn't really matter for a beginner though as the basic features work the same.
As for a book, I recommend "Expert Oracle Programming." You can read my JavaRanch review of this book.
I am assuming you mean the Head First SQL book... while I can't speak to it... from what I've gleaned from this board, it is likely a good book to start with... SQL is not really that difficult... it is just like learning math, algebra, and/or geometry... if you learn the rules... the result set will always be the same.