Configuring & Setting up Asterisk

Hello community,

I have followed ,I have installed asterisk in virtualBox for now.
I am able to login in to console.

Now I want to create a simple PBX in my local network.
I have 5 machine in my network.
I am thinking of assigning each a soft phone.and an extension no. now when ever any of these dials extension proper user will be connected on line.

At the advance level I want to assign a no. so that any one from PSTN can also connect those 5 pc using extension.

How to proceed?

I am very very newbie to asterisk.
Please explain the path stepwise also mention ifI require any special hardware to configure PSTN with it.