How Do You go from Idea to Actual Invention?

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I have an idea for an invention and would like to know a few things. First, how do I find out if my idea has already been invented or not? How do I do market research to find out if my idea is even worth inventing? Who do I contact to help me with the invention? I only have the idea and it quite possibly may take some computer knowledge. It's something that would be used in cars. What it is isn't important at this point :-) Are there legitimate companies that are available to help people in this area?