you can either use the hole command or draw a sketch and extrude it. or draw a circle sketch as a reference and use the hole command alligned with this sketch(will give you options for counter bore and counter sink etc).
When you create a new sketch and it asks you for a plane you probably dont have anything to click on, but if you navigate your tree on the left side you should be able to see your X Y Z planes and Datum axis for each. You can select your plane from there. draw your circle where it needs to be vertically, then when you extrude it you can give a start and end point etc. there are several ways, its going to come down to the detail you need and how comfortable you are with the modeling.