Typically, a search of the prior art is conducted to determine whether an invention meets the novelty and non-obviousness requirements for patentability. You can search for prior art online or at a patent depository library. However, a good search generally takes considerable time to complete, unless you choose to hire an attorney, or a service such as LegalZoom, that specializes in patent searches.
Because Provisional Applications for Patent are not examined for patentability by the USPTO, you can secure a filing date with a Provisional Application and then use the one-year "patent pending" window to conduct a more detailed search.
Your Non-Provisional Patent Application should always emphasize the differences between your invention and prior art. In addition, you should describe the history of inventions in related fields. This history will help explain why your invention would not be the next logical step in the evolution of the existing products (i.e., why it is non-obvious).
In addition, remember that patents are not limited to completely new technology. They can also cover significant improvements to (or uses for) existing inventions. For example, contrary to popular belief, Thomas Edison did not actually invent the light bulb. Instead, he improved upon a 50-year-old idea and patented the improvements.