Level 20 is the experience level cap. To ascend higher, you begin to acquire and level up armor with 'light'. The more you play, the more gear you'll get. Eventually you will need to start wearing legendary and exotic gear to continue to rank up. If you aren't lucky enough to get good drops, be sure to rank up your vanguard past 3 and purchase your gear with Vanguard marks! Same principle applies with Crucible!