Khan Academy videos are always very detailed and there are plenty of exercises you can complete on the Khan Academy website. "MathAntics" (spelled as one word) is also a great source for easy-to-understand videos on fractions, percentages, exponent rules, basic algebra and other subtopics you'll encounter on the GED math test. A quick Google search will turn up plenty of online practice tests. (These vary in length, subject matter & overall usefulness, so you'll want to try more than one.) Last but not least, invest in at least one GED test preparation book from a respected publisher. The folks who make all the books "for dummies" have a GED for Dummies book. The Princeton Review, Kaplan, Mometrix and many others also have some great test prep guides.
Best of luck on your test. :-)