The spouse of a US citizen may apply to become a US citizen after 3 years of permanent residency. A lawful permanent resident, not married to a US citizen may apply after 5 years of permanent residency.
The process to apply for US citizenship is called Naturalization. The fees needed to apply with form N-400, effective October 2, 2020 are as follows:
Effective October 2, Fee increases to $1,170
By Fred Wahl