I need help on number theory. It seems that I cannot do most of the questions despite the effort i put in and number theory is one of my major headache. Sometimes i couldnt even understand the solution even with the steps given. Need guidance.

There are several ways to improve on this topic:

1. GMAT Courses & Tutoring Worldwide:
http://www.800score.com/gmat-courses.html
Clearly, tutoring is the most efficient way. A professional, who can see where you're struggling, will provide the best guidance.

2. GMAT prep guide:
http://www.800score.com/content/guidetc.html
The prep guide is very handy and can give you an insight on the specific unclear moments in solutions.

3. Practice in solving.
http://www.800score.com/download.html
http://www.800score.com/forum/
Break questions into simpler concepts and you'll see which concepts you do understand, and which ones you don't. Keep you head up and start from the simplest questions and concepts, then move on to the hardest ones.

Of course, the combination of these options & your personal effort will give you the best results.

i have problems with number theory too.I dont now the necessary theorems and when I learn these theorems,
I cant use this.So I think you must firstly learn the basic theorems,then you can study with the hard problems.