Firstly, WELCOME to Apple world :)
Now, this is not that hard, and you should think it's quite easy.
if you never worked on a Mac before, you should invest some minutes to see some basic stuff, how it used to work on Windows and how it does now on Mac, for example, take some nice video tutorials from Apple website
go to Apple.com -> click on Mac -> down there there is several video tutorials, see some.
Regarding UPGRADING your Mac Mini, you should check what model you have and have a look on MacSales Videos on how to replace the hardware
you will find all models in the Mac Sales Tech Support page
About what hardware you should fit into your new Mac Mini, using Mac Sales you can order the memory, harddrive (or SSD) and any Optical drive as well, if you outside US, (though I'm in Denmark, I buy stuff from them time to time) you grab the model number of what you wanted, and "fly" to a near store and buy it :)
I hope it helps.
You are trying to buy 8Gb of Ram, and your mac only support up to 4Gb, in witch 3Gb is addressable.
Please read the specifications of your model before you buy anything!