The soft light cap is 900, which appears to already be reached for you.
For gaining light levels after 900, you need to complete activities that reward "Powerful Gear". These activities can be redone every week after the reset to get even more powerful gear.
There are three tiers of powerful gear. Tier 1, 2 and 3. The higher the tier, the more powerful gear you will receive.
You can find all activities in Shadowkeep that reward powerful gear here.
A lot of people don't know this, but you get 3 times the amount of drops per week by having three characters, one of each class. Having multiple characters of the same class won't drop more gear, as it is class based.
Once you hit 950, which is the hard light level cap, you will need to pick up "Pinnacle Gear" to continue your journey to the Pinnacle level cap (960). This is the highest current light level possible in Shadowkeep.
Pinnacle gear is dropped from the raid (Garden of Salvation), high level nightfalls, and other end-game activities.
Edit: I forgot to mention this initially, but there is a new seasonal leveling system in place.
At the start of every season, you will be given a seasonal artifact. As you level up the artifact with EXP gained, you will gain an additional bonus power level up to +12 power. This technically makes the max possible light level in Shadowkeep 972 power (960 max for gear, plus the +12 bonus light level with the seasonal artifact at max level).
The only downside to this, is that your power level bonus will expire and have to be re-leveled with your new seasonal artifact whenever a new season starts.