I want to install a side fan inside my computer and i was wondering if i should install it in a way it blows the air from inside the case to the outside or sucks the air from outside to inside the case and why??

Thanks.

If it's mounted low, have it pulling air into the case.
If it's mounted high, have it pulling air out of the case.

Cold air is more dense, than hot air.
As the cold air comes in the bottom, it's warmed inside the case, and begins to rise to the top, where your exhaust fans can draw it out of the case.