I'd definitely look up what to do before joining a LFR really as a tank. It's a real pain the ass having a tank that doesn't know what they are doing as it's quite a prominent role. I'd follow everyone else's suggestions with maybe going in as DPS first to get a feel for how the encounter plays out, then look at tactics that you'll likely understand much better now that you've seen the inside of the raid. Other than that, no, tanking is not something you should be scared to do (: