In the US, for a petition. to be legal it must be submitted to the appropriate authority who can take action on it. It must also not request for anything illegal and comply with all relevant local and national laws. It must be signed and those who signed must be qualified to do so. It must have a clear purpose and call for action. Everything it contains must be accurate and truthful. Often for a petition to be legal it may need to be submitted within a specific time frame, depending on what it's asking for or to happen.