Trying to make an Unreal project but some things aren't working

So, there are a couple things wrong with my project, one: My projectile just goes through everything, but I want it to be a physical object, two: When I open the Third Person Character blueprint, the only node that shows up is “Construction Node” nothing else, and when I try to search for left mouse button for the firing, it won’t show up.

If all you are seeing is construction node then it seems you are looking at the construction script function rather than the event graph. Look at the top of your window and what does it say?
You can select the EventGraph on the left hand side of the editor under ‘Graphs’ in the MyBlueprint Tab
Is that showing or not?
The construction script will not show you input keys because they are not allowed for construction. They will show when you are looking at and working with the correct graph.
Your projectile which I assume is a static mesh, will become a physical object when you give it an collision box or sphere or whatever shape you want. And then you must set up collisions so it will interact with whatever you want it to interact with.
Good luck. Do some reading.