Just use your Q to last-hit minions, and if you can do it without getting harassed, use your E to wipe out minion waves. Once you get your brutalizer you should be able to take out waves just by hitting them with the full duration of your spin.

You excel at harassing in lane with a partner. Ideally you want to get with someone who has a stun or even a slow. Lux works very well with Garen. Sit in a bush and wait for an opportunity to Q over to them then spin in their face. Just keep waiting in bush when you're not farming, your passive will keep your HP up.