It isn't difficult to guard as a CT at all. The place you spawn at is already a site, and if you wanted to move to A it would be a short walk up CT stairs or through North hall.
There also aren't as many entrances as you think there are, to go to B site you only have 2 maybe 3 entrances: West Hall, North Hall, or CT Stairs (only if A has been taken, there are usually CTs watching CT Window so usually this isn't a problem). Post 1-2 CT's watching North Hall from CT Spawn and place a few watching West Hall/Parking Lot and you have B under control.
Now for A Site, you have the same amount, 2 or 3: You have an entrance from North Hall, East Hall, and possibly from CT Window. Same thing as B site really, put a CT or two on North, put a few on East, and CT Window should be taken care of since we have CTs at West and usually there are a couple CTs up in the Window overlooking courtyard.
These are just theoretical examples of what to do, as your teammates don't always listen to you or they may just be new to the map and don't know where to go or what to do.
I would say if you were playing, go where more people are needed to cover a site. If there are 10 CTs and say 7 are watching A site or are in courtyard, then it would be wise to stay in B.