While many companies sell GED preparation courses (including full-length practice tests) that can be taken online, you can only earn your GED certificate diploma by taking the entire exam at a test site officially recognized by the American Council On Education (see the "Resource" section below for a link to the organization's website). Beware of scammers claiming to offer GED certification online, as these claims are categorically false.
The GED exam is more concerned with measuring your grasp of decimals, fractions and real-world word problems than how well you can perform long division. However, modern calculators are powerful enough to be used as a "crutch" for certain algebra and geometry problems. As a compromise, the questions dealing with arithmetic are put into the first part of the math exam, while questions where a calculator could potentially help a student are put in the second part. Calculators are allowed in the first section only.
Because "cheat" programs can be downloaded and transferred to many types of graphing calculators, the GED only allows you to use the Casio fx260 calculator, given to you on the day of the test. Even if you already own a Casio fx260, you are not allowed to bring it to the test. Only GED-issued models are permitted. (See the "Resource" section for a helpful video on how to operate the Casio fx260 calculator.)