Your GED math test will cover the following:
General or "Basic" Arithmetic - This includes knowing the order of operations (PEDMAS), working with negative numbers, rounding off numbers correctly, adding, subtracting, multiplying and dividing fractions, and working with decimal numbers and percentages.
Applied Arithmetic - This includes "setting up"/"translating" word problems, finding the mean (average), mode, median and weighted average of a range of numbers, understanding ratios & proportions, solving unit rate problems, using charts and graphs, working with exponents & roots, writing numbers in scientific notation, calculating probability & understanding combinations and permutations.
Algebra - This include one-variable & two-variable equations, inequalities, working with polynomials & quadratic equations, factoring quadratic equations, learning to apply the quadratic formula and understanding functions.
Geometry - This includes knowing the rules & formulas that apply to lines, angles and shapes. You'll be asked to calculate perimeter, circumference, area, surface area and volume, graph points on a grid, calculate the slope of a line (when given two points on the line), know how to write and apply the equation of a line in its "slope-intercept" form & interpret graphs in other ways (such as determining the "axis of symmetry" or the "period" of a graph.
It's not as bad as it sounds, but you'll need to be patient with yourself. Take the time to review the most challenging concepts until they finally "stick" in your brain. Although you'll want to pick one main textbook, sometimes you'll need to see & hear a concept explained in several different ways before it really "clicks" for you. Use multiple resources (especially free ones) to get as much info as possible on each concept until you feel confident enough to take the test. Best of luck! :-)