Despite what a lot of people say, she has a decent skill floor and one of the highest skill caps in the game. The thing I love about Riven is that she teaches you how to trade effectively in lane -- you can't get away with generally unnoticeable mistakes like you can with other champions. (i.e. engaging in the middle of an enemy minion wave, playing around CC cds, knowing your matchup, etc) A lot of this just comes from experience with her.
Granted, Garen's a pretty tough lane for her. If you're just starting to learn her, I'd just max E first, absorb his silences and farm up for teamfights and roam if possible. Once you get the hang of it, you can start trying to bait his abilities, and trip him up with the 4 second Q trick.
You should crap all over teemos in most cases. Just remember to grab a red trinket and don't engage on him if you know there's a shroom there. Once you close the gap, it's game over for him.
Also, learning how to quick combo with her (i.e. pull off the Q-AA-Q-AA-Q-AA roughly 30% faster than normal) gives a big leg up. It can take anywhere from an hour or so to a month or two of practice to get it down. Best thing is just to queue up and practice on the gromp until you get it, then practice it on bots. (using it on moving targets is so much harder)