Welcome to the world of exploits and penetration testing. There are so many different methods and ideas here that I could never fit them all in one post, but I'll give you a little analogy.
Imagine that your target computer is a house. All houses have an address which allow you to find them, and on the internet IP addresses do the same thing. A house has to have ways in and out, or the owner wouldn't be able to use the house. In the computing world, the front door is like the logon screen of the house, and the windows are like ports on a computer. A skilled house thief uses their eyes to look for weaknesses in the house; we use port scanners and vunerability scanners to the same effect. No-one would leave their front door or windows wide open, and firewalls and other hardening products lock up the computers on the connected to tinternet.
I would go on, but I've ran out of metaphors
Hopefully you gained something from that, and I'm sorry if I said loads of stuff you already knew.
There are some tutorials in the tutorials section that might interest you; stay tuned as there are more to come.
Have fun
