In other words.
Your computer wont notice a firewall in operation.
Most users won't notice the firewall software much because they don't run servers.
Don't load software when you don't have good reason to trust it.
Firewall software doesn't usually require that you know anything. It's not worth it to learn to do this "by hand" for most people.
Leave it on, because "pretty good is a lot better than zero"
About the only exceptions you're gonna need from filesharing software is bittorrent type stuff. The temptation will be to switch the firewall off rather than configure it right.
Don't. You are broadcasting your IP all over. If you ever needed security software, it's when you are making yourself public, as with gameservers, bittorrent etc.
You'd be amazed at how many computers are wide open. Type in their IP and you're looking at their hard disk. No router, no firewall