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)
... and more
What's Inside GMAT Pill?
Zeke Lee, GMAT Pill Study Method (Study Less. Score More.)
GMAT Pill Reviews | GMAT PILL Free Practice Test