So I reached level 20. How do I start getting gear with light?

By mindlessly farming that takes no skill and is complete luck

Sorry that not everyone likes the same games as you.
Not every game can be as good as MCC..oh wait

Getting better gear with a stat called Light. Each level you go up is called a light level, you can go down in light level if you swap gear with lower light, which some people don't realise somehow.

Did my last sets of hard raids last night to try n hit 30 before expansion.. No raid gear or weapons.. All ascendent materials... 3 lvl 29 n 3/4 chars!!!! Fuck destiny!!!

Do every daily vanguard bounty until you hit vaguard level 2. Then you can purchase legendary armor from your class vanguard.
Other than that, do the strike playlist for better rare items and hope for a lucky legendary drop.