For decorating cakes you will need either an icing surgyrne or icing bags. Lakeland do good sets with different attachments for making flowers and things. You can buy ready to roll icing. to ice a cup cake roll the icing out and cut out a circle using a cookie cutter, slightly dampen the cake or spread with jam. put icing on top, smooth over an then pipe a design on top. My mum has done wedding cakes in the past (including mine) and ive done it along aside her and on my own. look in you library for books before you buy any.
hope that helps. have fun being creative.