Sound Effects: Making ambient sound can be different from making effects for movies. But it all comes down to making use of your digital audio workstation (http://getinstrumental.com/best-daw-music-production-software-for-recording/). You can record live sounds with a microphone or use a synth/ instruments (vst plugins) for digital sounds.
But if you just asking how to create music on a computer: try a free daw like Reaper or Audacity in combination with some free plugins.
I think this will get you started.