well, in your own words, you’re a newbie. so off course, you’ll make tiny errors here and there. we all did, and we all do. it gets better 
I wonder, do you use groups / components ? if so, you can use that as a trick :
you can select all. you see I have Ty, a component, and 3 grouped boxes. and some random ungrouped stuff.
then, I’ll right click on something and go to select / deselect faces and lines.
what remains is all the groups and components.
a third right click / select and invert selection.
now all that is selected is the ungrouped / uncomposed geometry, ready for deletion.
in the go version there is an outliner panel where you can hide all the groups and components manually, leaving only the raw stuff. this is the free method