You don’t ‘register’ a patent. You apply for a patent using a patent application, and if you prosecute properly, the United States Patent and Trademark Office will issue you the patent.
What is Patent ? How to file a Patent ? What is needed to file for a patent ?

You need a description, a set of claims and figures (optional) prepared for your invention. And you need some money to pay the fees.
Optionally, there are people preparing these documents for you based on their experience and knowledge. They are called patent attorneys.
And, you need to be resident/national of a country having a patent office. It is better if you are a resident/national of a country recognized by WIPO (practically all countries you can name).
You need a checkbook, credit card, or debit card.
Government patent offices grant and issue patents. There are fees for that.
Even before it issues the patent, many patent offices publish a notice that a patent will be issued. There are businesses that look for these notices and the notices of actual issuance.
These companies then send you a very official notice so you can pay for “registering” or “listing” your patent. The very fine print may state that the document is not a bill. Sometimes, the document will admit that it is not coming from a government agency. Sometimes, the document will state what the actual service is: Recording your patent in their database, or maybe writing it on a scroll that they keep and maybe file with a copyright office on a periodic basis.
Warnings about these solicitations can be found at many patent office web sites, with page names such as:
  • caution-misleading-notices
  • pct_warning
Some of the businesses sending these solicitations have names such as:
Central Data-Register of International Patents
Commercial Center for Industrie and Trade
CPD — Central Patent Database
CPTD — Central Patent & Trademark Database
Euro IP Register
European Central Register of Brands and Patents
FIPTR — Federated Institute for Patent and Trademark Registry Inc.
FOIP — Federated Organization for Intellectual Property
IBFTPR — International Bureau for Federated Trademark & Patent Register
IBFTPR Service — Register of International Patent Application
IBIP — International Bureau for Intellectual Property
IIP — International Intellectual Property Office
Institut of Commerce for Industry, Trade, Commerce
IOIP — International Organization for Intellectual Property
IOPR — Intellectual Office Property Register
IOPTS — International Organization for Patent & Trademark Service
IP DATA — Register of International Patents
IP Direct — International Patent & Trademark Directory
IP World — Registration of International Patent
IPRS — Intellectual property register services
IPT PATENTS — Register of International Patents
IPTA — International Patent and Trademark Agency
IPTD- International Patents and Trademarks Database
IPTG — International Patent and Trademark Guide
IPTI — International Patents & Trademark Index
IPTO — International Patent & Trademark Organization
IPTR — International Patent and Trademark Register
IPTS — International Patent and Trademark Service
IPWTO — Intellectual Property World Trade Organization
IRO — Intellectual Property Office
IRO — The Patent Office
IRPT — International Register of Patents & Trademarks
IRTP — International Register for Trademark & Patent
ITMORG — International Trademark Monitoring Organization
ITPD — International Trademarks & Patents Database
Novislink limited
ODM — Office Data Management
ODM — Patent & Trademark Office
ODM — Patent Trademark Register
ODM — Register of International Patents
OPT — Organization for Patents and Trademarks
PATR Servis — Worldwide Patent Service
REGIPAT — European Register of Brands and Patents (REGIPAT)
Register of International Patent Bulletin
Register of International Patents
Registre des données bulletin européen des brevetes
RIPT — Register of International Patents and Trademarks
RIPT — Registration of International Patent
RPT Servis — World Wide Patent Service
UPTS — Universal Patents and Trademarks Service
WBIP — World Bureau for Intellectual Property
WBIP — World Bureau for Intellectual Property
WDTP — Worldwide Database of Trademarks and Patents
WIPD — World Intellectual Property Database
WIPD — Registration of International Patents Intellectual Property Office
WIPO — World Intelligent Property Office
WIPP — World Intellectual Property Publisher
WIPT — Registration of International Patent
Wirtschaftsinstitut für Industrie, Handel, Handwerk AG
WOIP — Globex World Organisation Intellectual Property
WOPD — Worldwide Online Patent Database
WPAT — Registration of International Patents
WPT — World Patent and Trademark Service
WPTI — World Patent and Trademark Index
WPTO — World Patent & Trademark Organization
WPTR — World Patent and Trademark Register
WPTS — World Patent & Trademark Service