Hi Daikrys, and welcome!

Well if i understand well, you just have to implement a sphere to sphere (or cylinder) collision.

To see if two spheres are colliding, you just need to calculate the distance D between them, and then check if D is smaller then the sum of the two radius:

if r1+r2 >= D then collision!

This apply for both 3d and 2d (circles).
Now, if you want a sphere to slide along another (if i understood correctly), you need some little complex calculation.

I'm not too good with this.. I'll try to read the tutorial and see if i get it

But is your world truly 3d? Becouse if the character can just move in four direction on a plane, then you can just use 2d circle collision.

Bye!