Hello
Which version of Solidworks are you using?
Normally, you just click anywhere on the sketch plane and then add the necessary dimensions. Be careful not to click on the sketch mark because it will constrain your drilling on this marker. The constraint would then have to be removed.
If you are new to solidworks or CAD, I advise you to start with the tutorials integrated into the software (see screenshot)
2017 version.
Hi, you just have to select the "Smart Dimensioning" tool when you draw your sketch
and you select the 2 entities to constrain your sketch and the diameter of your part and then you insert the desired dimension.
You can also go through the drilling function in mechanically welded tools but for it it's a little more complicated. I also advise you to first practice on some tutorial offered by Solidworks.
Personally, when I started, I took 2d plans of the companies I worked for (and yes, when I worked in the workshop, we didn't have a draughtsman in the company, everything was drawn by the boss and with a hand like a pig ... ) in short, I stole plans and I kept busy. You also have a lot of things on YouTube.
Could you take a screenshot to accompany your question?
I think it's a hole whose axis is perpandicular to the axis of the cylinder.
In which case, we just need to create a tangent plane and position the hole in that plane.
To do this, the simplest in my opinion would be to create a median plane to the planes that represent the X and Y axes, then to create a plane offset from it which will represent the background of the chamber.
Or else a removal of matter with revolution.