One you hit level 20 you no longer advance your rank by getting XP from missions etc. You need to find and equip legendary armour items (purple items). These can drop during play (rare though) or be bought from the vanguard merchants in the tower. Or won from completing some strikes / nightfalls. Or levelling up your rank with Vanguard reputation or other factions. And other in game activities. Equipping these will raise your light level. Then upgrading them will raise it more.