To patent a new idea the first step to make sure you have a NEW idea. If your invention or idea has already been created, or if it you have just tweaked someone else's patented invention or idea than it is not a new idea. In these circumstances you will not be able to obtain a patent for it. To check do a patent search at uspto.gov.
Once you have determined that you actually can patent a new idea go to the HSPTO website and find all the information you need for obtaining a new patent. Go to uspto.gov and download the patent application .and the patent filing procedures.
To patent a new idea you can either file it the application yourself or hire a patent attorney. Patent attorneys are expensive so have to ask yourself is the expense worth it. Make sure you your shop around and get references for patent attorneys.