You can go to the Apple Icon on your menu (top left) and under it you'll find About this Mac It will give you your basic info on your OS and hardware. To get more detail click the More Info button.
Before you upgrade your OS you need to first make a backup of what you have now. But before you do that ask your self is my current hard drive big enough? If not think about replacing your current drive with a bigger unit. In the process of upgrading it makes sense doing this now. Also think about how much memory (RAM) you currently have.
OK, you either are replacing your current HD or need to make a backup of what your currently have on your system:
Ideally you should get an external Firewire hard drive of equal or larger size of your internal drive. If your upgrading your internal drive you'll need to get an external case to hold your new drive in (later we'll use this case for your old drive). In any case connect the external drive (or case holding your new drive) onto your computer then using the migration assistant program you have in the utilities folder in your application folder make a full copy of your current system. If your upgrading your drive go into preferences and make the externally held drive as your boot disk and restart your system. If you find it the top listed drive then your ready to swap the drives review the iFixIt guide on how to do this.
OK, you have a back up and are ready to get the new OS installed. Before you do this make sure you have a good connection (and fast) to the internet and have your systems battery fully charged and you still want it connected to a charger that is plugged in.
Launch the App Store application using your credit card or iTunes card/s buy the OS and install it. But, before you run it I would recommend making a back up copy onto a USB thumb drive. The OS install program will walk you though the rest of the steps.