Caulking and weatherstripping are both easy and effective air-sealing techniques that can provide quick, money-saving results.
Caulking: Caulk is typically used to seal cracks and openings around stationary doors and window frames.
Weatherstripping: This is used to seal moving objects, such as operable doors and windows.
Further testing: While these techniques resolve obvious air leaks, there are likely more gaps that you can't see. A home energy audit and blower door test can help identify these areas.