The "bible" for GMAT preparation is the Official Guide - it's a big orange book full of actual old GMAT problems. The level of difficulty is very accurate.

You'll find that the math required on the GMAT isn't all that advanced - it's the application of basic math principles to new situations that is tested. That said, you need to have a very solid foundation in quick mental math and basic algebra. If you're uncomfortable working with algebra, you might want to get a non-GMAT-related algebra workbook or textbook to provide that foundation. The more solid your fundamentals are, the easier it will be to add the complexity of GMAT problems.

Come back when you have more specific questions - good luck!