Members of the House of Representatives (only representatives can introduce bills). Before a bill is introduced, it is typed on a special House form and signed by the Representative who will introduce it. Any member can introduce a piece of legislation, it is handed to the clerk of the House or placed in the hopper (box where members deposit bills).
A bill is introduced in the House of Representatives (one of the two chambers of the Congress); committees hold hearings/markups; the whole chamber votes on it; it is passed to the other chamber for a vote, basically.