Riven is a champ that is extremely good if you are mechanically skilled, not something I'd recommend if you are new to mechanical champions. How to get better with Riven though? I'd personally suggest watching VOD's of Riven players (BoxBox, Faker etc). Also read up about cancelling animations > different combos.
For example you might find these useful.
There is several tricks with Riven, just search for it and practice, if you can master it you become a *** ingame