On your computer, open the Settings app and select System in the upper left corner. This opens the Display page. Scroll down the page until you see a selection box for 'Multiple displays'. Select 'Duplicate these displays'.
Use the on-screen menu on the TV and find 'Input' or 'Source'. You'll see a number of input slots. Scroll down and select them one by one until you get both video and audio. Some TVs also have an input option called 'PC' or 'Computer'. This isn't always the best choice, but try it anyway.