Sometimes a patent is not the way to proceed... Depends on what your invention is. Do you want to license it? Manufacture it? Start a business? How much will it cost to produce? What will the market bear for a price point? How will you distribute it? What will you claim as original?
Don't bother with a design patent... you should get a utility patent. Patents can range from around $5k to $20k or more... depends on your atty and the price structure. Sometimes it is worth hiring a really good atty because if the claim is written with some kind of "loophole" that allows someone to legally knock it off, you just paid for a worthless patent and exposed the entire idea to the public. I have been hired to break some patents and the way a claim is written means everything.
You need to check the USPTO online. Do a search yourself first... its not that hard. What do you plan to do with the invention? You can also apply for a provisional patent which will give you some protection for a year (?) from the time you apply to go to a full patent. It is cheap and will give you time to see if there is interest in the market to sell your idea. Do not jump in and patent it until you figure out what you want to do with it and if you have the money to develop it whether licensing it or manufacturing it. However, if you ask a patent lawyer, chances you will be told to proceed with a patent.
Remember that a patent only says when you came up with the idea and what is original about the idea i.e. your claim(s) for the patent. If someone knocks you off, you will need to hire a lawyer to protect it meaning you will have to spend some money to go court etc... If you license it and hold the patent, and someone knocks it off, the licensee will most likely require you to protect it meaning you will be suing whatever company is knocking you off... and this can get very costly esp if the cost to protect it is more than what the patent is worth.
There are a lot of people who patent ideas all the time but the majority of them never make it to the market. I think the percent of ideas that don't make it is around 70%... might be an old number but I don't think its changed much. I have several patents that I have been involved with and have also been hired to design product to get around existing patents. I'm an inventor/designer so I have experience in this field but I'm not an expert on patents (like an atty) but I am an expert on product development.
And by all means protect your idea, do not expose it to anyone unless they sign a disclosure form. Good luck with it. Feel free to PM me if you want more info or have questions.