Can anyone lead me to a resource to find laws about running a contest online?

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Was looking for information when it comes to running a contest within the US that doesn't involve any sort of payment to participate in the given contest. Needed really, any information on it that would be important to know. Such as the types of countries that would be able to participate in it, the taxes relating to it, etc. etc. and all this information I need to know in connection with the contest being run online through the US. Thanks to anyone who can help.