It is really very simple.

Google Buttercream icing and you will find many recipes, any and all are good really - it is simply butter and icing sugar, colouring if you want it.

In terms of piping, it will depend if you want the star nozzle or the ound (or the dog poo nozzle ;p) and also in terms of size of nozzle, what size your cupcakes will be... IE A larger nozzle for larger cupcakes will give you a nie swirl on top, which you can then sprinkle with a few silver suagr balls or the premade sugar decos from the supermarket.

HTH If you have any more specific q's feel free to ask...you may need to practice a bit with the bag first but it is quite simple, there is a technique but you should be able to find some you tube vids on it, it is a bit tricky to explain over the puter!