Hi Ekaterina,
Since you want to live stream on YouTube you must have an RTMP ingest URL which looks like rtmp://a.rtmp.youtube.com/live2/{stream_key} to stream your content to YouTube.
Live streaming is usually done with RTMP (Real-Time Messaging Protocol) so your video content should be pushed in FLV format only.
You can capture the screen or provide a video file as input to FFmpeg. You can check this link for the tutorial.
https://trac.ffmpeg.org/wiki/Capture/Desktop
Or visit: How can I serve multiple audio files continuously as a live stream using FFmpeg?