First, check the U.S. Patent and Trade Office website. You can do a quick search for your idea (use some key words or concepts) to see if anyone else has something similar. If there is something similar, unfortunately, you're likely out of luck enough to not spend the thousands of dollars to have a patent attorney do a full search.
If there is nothing there and you're still feeling really good about your idea (i.e. is there a market for it? Would people be willing to buy this? Does it solve a perceived "need"? otherwise, why patent it?!), contact a patent attorney who can do a full search for you and who can determine if there are pieces of other people's patents that may apply to your idea. This can run $2000-$3000.
If at that point, the lawer comes back and says you're good to go, you start the real paperwork with the patent attorney to file with the patent office. If you want to make sure the paperwork is done right, I highly suggest using the attorney to draw up the work. This whole filing process can run upwards of $10,000.
IF you don't want to go through all this hassle and you try to market your unpatented idea to a company to manufacture for you, there is nothing stopping that company from stealing your idea and manufacturing it themselves and profiting from all of your hard work!!
My husband went through this (costly) process with a fantastic idea only to have the lawyer find that there was a small portion of someone else's patent that covered what he wanted patented. So he was out of luck. Patents are pretty all-inclusive that way. It's not just the end-product or idea that is patented. It's all the building blocks that are included, too. Thankfully, we didn't get to the $10,000 stage before finding that out!!
Good Luck!!!