Streaming games for Pc is a bit ambiguous than streaming from an Xbox One or Ps4. Most of the computers do not come with an inbuilt software to get you started. Instead you will have to subscribe and install softwares like Xsplit for full operation. I would recommend downloading Open Broadcaster Software (OBS) endorsed by twitch which is completely FREE usable in both Windows and MacOS.
I have Illustrated the steps below to make you understand better:
Step 1: Download OBS
• Download the appropriate version of OBS for your PC through the Twitch interface.
• Click on the dashboard and then to the link labeled streaming apps.
• Click on the OBS and then to the Download button to download the software.
• For your convince I have already set up the direct downloading link. Click here
to download OBS
Step 2: Enter your Stream key
• Go back to the Dashboard in the Twitch website.
• Click on the Streaming key option which is just beside the Streaming apps.
• Click on it to get the long string of letters and numbers.
• Copy the key and return to OBS.
• Choose the Stream options and paste the key into the required field as shown below.
Step 3: Setting up the stream
• Click on the output tab in the OBS and make sure that your video bit rate is set to about 2500.
• This will lead to stream your content at 720p.
• You can increase this number if you want to broadcast it at a higher resolution.
• Set the audio bit rate to 128 regardless of your resolution. You can lower this no if your internet speed is slow.
Step 4: Set up your game for streaming.
• Plug in the microphone or any audio device which you want.
• Go to the sources menu in the OBS screen.
• Select audio input capture and click on the icon with hand and a piece of paper.
• It will give you the list of available audio devices.
• Choose the microphone or audio device which you want to use.
• Open the game which you want to stream and get back to the Sources menu.
• Then, select the Game capture or window capture and click on the icon representing with a Plus sign.
• Select your game from the list of possible options from the Drop down menu.
Step 5: Finally stream!!
• Hit the start streaming button in the main OBS screen.
• You will be able to begin broadcasting immediately from your Twitch account.
• Just click the stop streaming button when you are done. That’s it!!, you have done Game streaming for the Windows/MacOS.