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You need to be able to touch type - really, really, really fast. Almost always this includes the use of a 10 key pad, or the ability to type things that are not necessarily in a pure speaking language. This likely won't be human readable sentences.
If you want to practice, download a couple of java or perl programs off of the web and practice typing them in at speed. Then grab a copy of the yellow pages and practice typing every phone number in.
When you can reach 90-100 words per minute (plenty of free typing test tools out there with this type of measurement capability) - you'll be able to pass any capability test they can throw at you during the interview process.
Abhijeet made a good point that you need to obtain ba asic operating knowledge of Microsoft Office. I would also recommend that you explore Microsoft Access so you have some knowledge of how database works. Often, we only focus on the interface but believe me, try to get a better understanding of the engine under the hood will pay you dividends in helping you land the next senior-level job.
I think a basic understanding of Microsoft Word and Microsoft Excel will greatly help. Most data entry activities will be done through a computer and Excel/Word are commonly used applications.
Data entry is an entry level position - so it should not be difficult to shoot for. I would suggest practicing your typing skills and specifically your 10-key. Practice practice practice. Then practice some more. Once you have built up to a competent level start applying for data entry jobs.