Patents protect very specific aspects of a product, and you will not be able to patent anything that exhibits or performs any of the protected characteristics of another patent-protected invention. You can, however, patent something similar, though you may have to hire a lawyer to help write the patent application in a way that does not overlap with any existing patent.
Conduct a patent search on the U.S. Patent and Trademark Office (USPTO) website and find any patents similar to your invention (follow the link in Resources to the USPTO’s database).
Read the issued patents’ claims. The claims are the part of the patent that explains -- in very thorough and technical detail -- exactly what the invention does that is under patent protection.
Draft claims for your invention in a way that does not ask for protection for any of the same characteristics or processes of the similar invention, and submit the patent application.