Magento - Function calling on an Event trigger

i want to call a function when a customer signs in. For that i need to create a function when an event is triggered ,here in my case event should be "customer_login". Can an开发者_C百科yone please help me by providing some sample. i have came across so many tutorials .As i am a newbie am quite unaware of magento file structure .please help

Please see Customize Magento using Event/Observer. Especially the example steps 3, 4 & 5 which explain the files to create.