I have a microphone with a 3.5 mm jack. Can I use this for recording in garage band in my ipod touch. Thanks for the help.
Not directly - you need, at minimum, a plug adaptor to take the 2-way connector on the mic to the Apple standard 4-way connector.
There are a lot more advanced methods too, involving actual 'live' interfaces designed to do just that - it really depends on what kind of mic it is as to what interface would be best, though if its default connector is a 2-way mini-jack, then my guess would be it's a non-powered electret condenser, which are not always the best for music recording.