To make the most of your efforts, you'll need to be able to type quickly and accurately. Depending on the type of data entry involved, you may need to be comfortable using your keyboard's number pad.
In addition to fast and accurate keying abilities, you'll also need to be able to read quickly. You might be pulling data from a spreadsheet, document, or even handwritten notes.
Many online data entry jobs are paid by the number of correct keystrokes. So the greater your ability to quickly read/scan something and key it, the more money you'll make.