There are lots of ways to do this. What best suits your gameplay?
Examples include:
Damage based on your level (i.e. attack number = level number, not usually recommended, but good for a very basic game).
Dynamic stat system increased per level (+2 attack, +1 defense, etc.).
Normal damage system that doesn't change, based on your and enemy level (your level being 15 reduces your damage against level 20s, preventing you from skipping ahead to harder content).
Upgrade system, based on items/skills you find.
And so on.
Examples include:
Damage based on your level (i.e. attack number = level number, not usually recommended, but good for a very basic game).
Dynamic stat system increased per level (+2 attack, +1 defense, etc.).
Normal damage system that doesn't change, based on your and enemy level (your level being 15 reduces your damage against level 20s, preventing you from skipping ahead to harder content).
Upgrade system, based on items/skills you find.
And so on.