system intrusion through the internet/wireless internet an..

I am very new to networking stuff, and I want to setup LAN at home.
I worry about computer security. For example, if my home machines are
connected to the internet, is it possible that hackers can get in my
machine's file directories through the internet? I know it is possible,
it matters how easy they can do it.

In what situations they can do that, normally they should not able to
do that, unless they install some spyware inside the machine, is that
correct concept? and what we should do to protect our machines? If I
setup firewall and anti-virus, it that good enough?

If my home has wireless setup, will the system intrusion more easier?

please advise. thanks!!