Your Win Condition
Get a solid advantage over Camille at the berserker’s greaves spike so that Camille is out of the game. Once Camille gets Divine Sunderer your window to solo kill her has closed. Going 0/0 in lane is viable but dangerous considering Camille has equal if not better scaling than you in both the sidelane and teamfight.
General Strategy
Always avoid Camille’s empowered q2 damage. Avoid trading into Camille’s shield. Flash ulting Camille is risky when she has ult up. Garen is a relatively immobile champion and Camille has good gank set up with her slow dash and ultimate.
Trading Patterns
You should really look to trade when you get berserkers greaves. Your berserker's greaves are a much stronger spike than her sheen ruby crystal. Bait Camille’s shield out and then look for a quick q + e onto her. If she buffers her hook walk away and w the stun. Trades should be positive for you as long as you avoid the q2 damage. If she has grasp avoid trading one for one autos with her especially when she has grasp or sheen up.
Wave Management
Focus on farming and getting to your 1100 gold power spike. The wave position doesn't really matter too much early on as long as its possible for you to get a good reset when you get the gold. If she slow pushes into you then you can slow push on the bounce and she should not be able to break the push unless you are low.
Their Win Condition
Camille should be playing to scale to first item and to side lane for the rest of the game. Once she gets it, sidelaning only gets harder for you.