Research
There are some good patent databases available free on the internet. You can therefore make a search yourself to find out whether your idea is new or whether somebody else has already published the same idea. If you find that your exact idea has been described before, you will not be able to get a patent, but small differences can in some cases be enough to justify a new patent.
See our USEFUL LINKS page for links to searchable databases.
Get hold of an existing patent specification (e.g. from the searching site listed above). Read it so that you see the level of detail required in preparing a patent application.