retrieving itunes

hello. i have a complicated story, but please bear with me. i had an original iphone and the left the job where i set up my itunes account since i had no computer. the last update i did was in May of 2008. Around August of 2009, when i would hit the ipod button of the iphone, it would go to a grey screen like it was going to enter the ipod but after about 10 seconds it would just go back to the home screen. I recently purchased a iphone 3g s, which i love,as well as a macbook pro(love it as well) and i am trying to access my old purchases and i just cant seem to figure it out. should i sync with the macbook? anyone have any ideas? thank you