Congress.govis the official source for federal legislative information. On the main page, the default search is for current legislation. You can change the drop-down menu on the left to "All Legislation." Enter your keywords, topic, e.g., "immigration reform," "gun safety," etc. On the results page, use the filters on the left to limit your search to specific Congresses and legislation type, e.g., "Status of Legislation."
Both the House of Representatives and the Senate have lists of the dates and sessions of Congresses. Sometimes you may need the Session of Congress but only know the year when a bill was passed, not when it was first proposed.
GovTrack.usUse the left side to begin your search and enter your keyword(s). Then select a Congressional term and current status. Alternatively, select a Congressional term, scroll down and select a subject. You can also browse by subject at the home page by selecting "Bills and Resolutions" and then choosing the "By Subject" tab.