About the index.php

Hi, um, I’m pretty new with this Word press. I’ve installed it already but now I want to customize it, use my own layout but I’m not sure which parts that I can delete and which that I can’t. Can anyone tell me which parts of the codes must be included to ensure that my blog will work. And can I start on a new database and then combine with another one later?? Cos I have some other old posts in another host and I want to shift them over, only I have got the backup for the old database yet.
Thanks