Mono can't be learned in a day.
It does need practise.
You have taken this in RAW which is good.
I think the whites are blown a bit - you should be able to see detail
in shadows and in the whites. St. Paul's does look overexposed.
If you use Photshop or Elements you should convert to mono not desaturate.
Then use the colour sliders to adjust each colour in the image.
Go back to Levels, hold down the Alt key and check the right hand side of slider - the screen will show up
any areas which are blown in white. Similarly hold Alt key and check the blacks at the left handside.
Ideally you should have a full tonal range with a little white and a little black and every tone in between.
You can rescue the blown areas a little by using the burn highlights tool and bring back detail in shadows by burning