then, two or three observations:
1: In the meantime, in this forum, it is a good rule to present yourself and then ask questions: http://www.cad3d.it/forum1/showthread.php?t=1322: Use the search function and see if there's already a topic like that
3: What do you mean by horizontal? Maybe perpendicular to the axis?
4: is the axis of your hole disapproved from the axis of the cylinder?
Let me explain the simplest solution: in axis and perpendicular:
Therefore, your cylinder will almost certainly have the axis parallel to a plane of origin. Suppose the xy plan
you have to create a tangent plane to the cylindrical face and parallel to the xy plane on which you will make a sketch where you will insert a point that represents the hole entry point (or extrusion).
to make the plan click on the command "plan", then, in indifferent order, on the xy plane (in the browser) and on the cylindrical surface and of your cylinder.
You should have created the bribe plan.
create a sketch on the obtained plane,project the reference geometries you need and put the point in the desired position.
Get out of the sketch and open the "hole" command.
If instead of the hole you want to make an extrusion, in the sketch you will design the circle of the diameter of your hole, instead of the point....
