There are many ways to gain levels in runescape, unfortunately you have not been very specific about your combat or wether you are pay to play or free to play. some popular methods are:
- Pest Control - Soul Wars - Bandits - Slayer Tasks - Penguins
If you are looking to level up fast in free to play, there is no option but to mass kill monsters, depending on your combat level
(1 - 10 Goblins) (10 - 20 Dark Warriors In the Wilderness) (20 - 40 Hill Giants) (40 - 50 Moss Giants Or Deadly Red Spiders) (50 - 70 Lesser Demons) (70 + Greater Demons or Cockroach Soldiers) (80+ kill any high level monster you find)
I suggest using a method of renewable food to kill all of these monsters, such as catching fish at a nearby location or getting a friend to bring some for you and paying them.