To create Light, you need to defeat the first god, Hyperion. To defeat this god, you need to build up (mostly) your Attack stat. To do this, you need to create shadow clones that train their skills and defeat monsters.
At the start of the game, you will be creating shadow clones. Go to the Train section and assign clones to start training on the first skill listed in both the Physical tab and the Skills tab. Around 100 each is a good start.
Then go to the Fight section and assign clones to first the first monster on the Monster tab. Easiest is to just click the CAP button that will assign as many clones that are needed, but no more than are available. As your power increases, you'll need less clones to fight a monster and the number assigned when clicking CAP will start to go down.
Keep an eye on the Attack stat in the Stats box (lower left). Now go back to the Fight section and click the Gods tab. The first god is Hyperion. Hover over his name to see his stats. When your attack stat is a good deal higher than his defense stat, click the FIGHT button. The fight may take a while, you can wait or click Stop and try again later.
After you defeat Hyperion, go to the Build section, Create tab and the 'Create' button should have shown up next to 'Light'. Click it and voila. :) If it doesn't immediately unlock, close the game and re-open it (it unlocked for me when the game was closing).