watched lots of SC2 on teamliquid.net before i got in the beta.
mutas still get me though
As already stated here, if you know your opponent is going mutas, you need to really turtle your base up. As also mentioned, thors while effective against mutas, aren't particularly reliable because they move really slow, mutas can run circles around you. In addition, mutas can fly where your thors cant.
Do NOT build ONLY missile turrets in your base. I tend to build at least 2 bunkers in addition to at least 3-4 turrets. And build them in such a way they can attack anything attacking my SCVs. Otherwise you're going to get owned by a group of zerglings.
From the beginning, build in such a way to prepare for a zergling rush. Plan to have at least 1-2 marines out already by the time zerglings get to you. Then have your SCV's somewhat defend your marines. At least 4-5 SCV's easily keep the zerglings from massing around your marines and owning them. Then build hellions and harrass the hell out of him while you tech up. Properly microed hellions will absolutely OWN zerglings. And then proceed to work on his harvesters to minimize the effectiveness of zerg early expansion.
Once he has muta's you should at least have a few vikings. While you cannot meet him in open ground effectively, the vikings are more than enough to chase him out of your well defended base/expansion. Slowly build up a force of vikings and some marines with 2-3 ravens. I try to time it so that by the time I'm done building ravens, I can start building a few battle battlecruisers otherwise build banshees. You need air that can attack ground to avoid having to land the vikings while he still has mutas. Once I have a good set of units I go towards his expansion or base and set up a few point defense drones outside it, then attack the base to draw him out. Retreat to the drones and proceed to own him while his force does very minimal damage to you.
You have to stay mobile when playing against mutas, and keep bases defended to keep the mutas busy while you reply with your forces.