Power/Light depends on your equipment: weapons and armor. The maximum level is 300 (although this number isn't definitive and can be increased by the developers).
The best way (and practically the only one) to get high-class equipment that increases your Power is to complete Milestones goals. When you are close to 250 level it can be the only way for you to reach the maximum level. You should pay extra attention to The Weekly Flashpoints - they are the most profitable ones and it would be a sin to miss them - they reset on each Tuesday.
Another good method for getting new equipment quickly - sadly, when you do that you have almost no chances in getting a better gear, only a "replacement" with a similar level - is to participate in Crucible fights. Players' levels are made to be the same there and because of that you can participate in fights even if your level is low. What matters more are your own skills, knowledge about the gameplay and a good tactic.
If you want to get gear replacements with similar or a little bit higher Power value but more suitable for your play style then you should try to collect tokens (each planet has their own tokens). You get them by completing missions, side tasks, patrols, public events and clearing the Lost Sector. Tokens can be exchanged at NPCs found on a given planet (a character icon on the map) and doing that gives Legendary Engrams - this means that the items are random but at least they have a legendary rank. You can also find collectibles (in the form of crystals, glowing nets etc. - each planet has their own ones and they can only be exchanged on that particular planet) and exchange them as you do with tokens. However, they have a smaller value.