The Senate has 100 senators, 2 from each of the 50 states. In order to be a senator, you would have to at least 30 years old, have 9 years of citizenship, and be a resident of the state you wish to represent.
State Legislatures used to elect Senators to the Senate, which meant that the people chosen were often the more politically important people of the state. But this was changed by the 17th Amendment:
The Senate of the United States shall be composed of two Senators from each state, elected by the people thereof, for six years; and each Senator shall have one vote. (Two Senators from each state, elected by citizens for 6 year terms and one vote per Senator) The electors in each state shall have the qualifications requisite for electors of the most numerous branch of the state legislatures. (If you can vote in state elections, then you can vote for the Senator) When vacancies happen in the representation of any state in the Senate, the executive authority of such state shall issue writs of election to fill such vacancies: Provided, that the legislature of any state may empower the executive thereof to make temporary appointments until the people fill the vacancies by election as the legislature may direct. (If one of the Senators leave office, then the governor is allowed to select someone to fill in the state as long as the state allows the governor to do so)
The presiding officer is the Vice President of the US. If the Vice President is absent, then the President takes the VP's place.
Powers given to the Senate:
Act as jury when holding impeachment trials
Ratify treaties (requires two-thirds vote of Senate)
The President's nominations or appointments to high-ranking offices and positions must be approved by two-thirds of the Senate