Many online scams require you to pay money in order to buy their software or a book and anything of that nature. This is the first step in not getting scammed is to avoid paying any entrance fees or anything of that nature.
General Rule: IF you have to Pay STAY AWAY
Step2
If something you are looking at looks legit go to forums and do some research. See if anyone has said it was a scam. If a lot of people say its good, usually it is good and you can trust it.
Step3
A lot of Get Paid to Sites are great to do and provide a steady income, try them out. For some good ones look in the references section. They are always free and pay decent.
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