aiqbal,
Wow, 20 years since algebra/geometry! You'll need to review some materials before you take the exam so you can score above your threshold and get into the programs you want.
Usually a few methods students choose:
1) buy a book and self study
2) Enroll in a class
3) Enroll in an online study program
I encourage you to do your research and explore what works for you. Everyone has a different learning style and everyone may realize they may need resources in certain areas to help them get above a threshold in certain sections. You'll know better as you research what is out there. Of course, I always encourage students to check out GMATPill videos and read several of the articles on the blog (ie psychology, timing strategies, reading comprehension thought process, etc)