A baby can go flying after he/she is one week old. That's the general guideline.
I took my baby flying (from Paris to SG) when she was 1 month old and then again when she was 3 mths plus (back to KL) and again (SG to Bali) at 5 mths plus. Oh, she'll be flying again next month (back to KL) - yes, Eva has flown A LOT. :P
IMHO, covering your baby's ears will not help with the air pressure. In fact, it can make things worse for some babies. Swallowing does - so if you can, feed her during take-off and landing. If possible, let the pilot know that there is a baby on board as cabin pressure can be adjusted and it's his responsibility. Ignore what the air hostess all say about it being centralised. My dad was a maintenance engineer with the Malaysian Air Force and after speaking to him about it, he told me that cabin pressure is definitely NOT centralised or fully automatic. A pilot can adjust the cabin pressure if he wants to.
Take off is always easier to handle than descend so do note that. The last time I flew with her, we managed to get her to sleep during a good part of the descend (it starts way before the announcement - at least 30 minutes before ETA) and afterwhich, when she started fussing, I waved her toy and she got distracted with it. So having a favourite toy at hand - one with lots of sound or a rattle - is good. She didn't cry at all and didn't even notice that we were descending until we landed! (Unlike the time when I flew with her at 3 mths plus - it was horrible! The pilot was so crappy!)