Dear Ms. Doonis,
Thank you for posting your request on History Hub!
Death certificates are typically created and maintained by the political jurisdiction where a person passed away. For the United States, this would be the state or territorial government. The National Center for Health Statistics provides a list with links to the vital records departments of the various US states and territories. The US Department of State's Bureau of Consular Affairs maintains some information regarding U.S. citizen deaths overseas. The Social Security Administration maintains the United States Social Security Death Index which is an index of deaths reported to them beginning in 1962. Several genealogy websites provide access to it, to include FamilySearch which makes it available for free. Additional options include Find-A-Grave and obituaries in online newspaper compilations and some genealogy websites. FamilySearch has a wiki page on finding United States Death Records as well as additional pages for other countries.
We hope this information is helpful. Best of luck with your research!