There are two kinds of friction, which depend on the state of the object.
For objects in static state we have:
Ff = μsN
For objects in motion we have:
Ff = μkN
Where
μs: coefficient of static friction
μk: coefficient of kinetic friction
N: the force on the surface, which can be determined by using: N = mgcosθ
Where we have:
m: mass of the object on the surface
g: the strength of the gravitational field
θ: the angle of the surface