At least 30 years old, must have been a citizen of the United States for at least the past nine years and must be an inhabitant of the state they seek to represent.
In Article I, Section 3, clause 3 the requirements for a US Senator are outlined.
There are 2 senators for each state and the serve a six year term. Originally senators were chosen by state legislatures. When the 17th Amendment was passed, senators became elected by a direct vote from the people.