Most of the legit opportunities out there take time to build so if someone makes an outrageous claim of income it probably isn't true and those that are true sure didn't make that kind of money when they first began.
There are lots of sites telling you have to make big bucks online and most of them are scams. Data entry jobs are the biggest scams out there! They charge you for their "great" info, then you find out that you're stuck with $200 of ebooks and having to sell the same crap program to someone else. If you have half a conscience, this isn't for you!
This isn't the fastest way to make money, but it's an easy way that doesn't require you to do anything you normally don't do. The first rule of thumb is to never, ever pay to join a any website. There are plenty of free sites that you can join risk-free. I like odesk.
There are a few legitimate data entry companies but are not hiring and have large waiting lists (1-2yrs). The majority of data entry jobs you seen online are scams. A real company will want you to have skills fast typing, 10 key, may want you to know excel, and will usually ask that you have high speed internet. Once in a long while you may found one online on a job website but it is rare.
One option is to freelance and bid for data entry jobs. However, you are bidding along with many people and often other countries-so you may have to bid really low just to get a job and/or project.
You can become an independent contractor, but you have to have experience in the field of data entry, because you're basically starting a business which means you have to have all of the legal paperwork. Otherwise there is no such thing as work at home data entry.
So if you are starting I recommend search and learn first and thyen join website without any investment.