You have to be careful about pain and playing. Of course there will be some discomfort expected when you start, you have to build up callouses on your finger tips and dexterity on your tendons (fingers, wrist and forearm). However, you have to be careful not to give yourself a srtess injury. Make sure you keep the stress off the wrist by keeping the elbow out from your body. Safest thing to do is take regular breaks and have a good look at how you hold the guitar, I've only ever really played electric so I can't give exact tips on how you should be sitting but you should get good tips by looking at really good players. Hope that helps a bit.