How do I transfer ownership of a patent to a company?

The previous four responses all correctly point out that the transfer of patent ownership from an inventor to a company is achieved by way of an assignment of all right, title, and interest in that patent, which is recorded with the USPTO. The only thing I'd add is to consider whether you own the patent or whether it is owned by another entity/person. While it is true that ownership of inventions vests with inventors, if those inventions were conceived while you were employed elsewhere (and are related to the work done with that employer), it is likely that ownership transferred according to terms in your employment agreement (e.g., either via an obligation to assign or through an assignment in fact). And, if true that your ownership interest transferred to another company, you would no longer have an ownership interest to assign to your new company. Don't forget, too, that any co-inventors also have an equal stake in patents covering those co-inventions, and can assign/license their ownership interest to a party that is distinct from -- and maybe in competition with -- your new company.