I would focus on learning these 8 concepts first -- you can find good explanations here on BTG, on GMATClub, on Grockit's GMAT blog, and on many online math sites such as Purple Math. But these concepts are definitely going to appear on test day:
1. Number Properties. The properties of odds and evens, integers, fractions, positives, and negatives will all appear in various questions on your GMAT test. Don't ever make assumptions about unknown variables. Unless you are told otherwise by the limitations in the question, variables can be negative integers, negative fractions, zero, positive fractions, or positive integer. You may need to Pick Numbers from multiple categories before you can arrive at a correct answer.
2. Inequalities. Some of the hardest Data Sufficiency questions on the GMAT will involve inequalities. Usually they are combined with Number Properties to make them even more challenging. Remember that when you multiply or divide by a negative number, you must reverse the direction of the inequality.
3. Coordinate Geometry. Not only will you need to know the standard equations for a line, parabola, and circle, but also you will need to memorize the distance formula, the midpoint formula, the slope formula, the relationship between slopes and the different quadrants, properties of parallel, perpendicular, vertical, and horizontal lines, as well as the quadratic formula/discriminant. Coordinate Geometry is a big concept on the GMAT, so don't put it off!
4. Plane Geometry. Triangles are tested a lot on the GMAT. You should know the Pythagorean Theorem, Triangle Inequality Theorem, the special right triangles: 45-45-90 and 30-60-90, as well as the properties of isosceles and equilateral triangles. Other plane geometry concepts to review include angles, circles, and polygons. Make sure you know how to find the perimeter and area of all shapes, and be comfortable dividing irregular shapes into manageable pieces.
5. Rates & Work. The classic need-to-know formula on the GMAT is D = R x T, Distance = Rate x Time. This concept is most-tested in Word Problems.
6. Ratios and Proportions. A ratio is a relationship between two things. Given a ratio and one "real world" number, you can always set up a proportion to solve for the other missing "real world" number. Sometimes you will need to do this for similar triangles in Geometry, and sometimes in algebraic word problems.
7. Percents. For percent question with unknown starting values, always pick 100. Make sure to review profit and interest concepts and know the formulas for simple and compound interest. Remember to calculate percent questions carefully, step by step, especially in long Problem Solving questions.
8. Systems of Equations. You will need to be able to solve for a system of equations. Remember the "n equations with n variables" rule. If you have 2 variables, x and y, then you will need 2 equations with those 2 variables to solve for both. Familiarize yourself with Substitution and Combination.
