Really confused about where code goes - yes I'm a noob!

Hello everyone - I am a newby. I have been able to figure out how to install Atahualpa, but I am confused about how to customize things (beyond the simple options available). Please see below for a few of my questions, and thanks for your time! (I have checked out 'wordpress for beginners' on their site, but I am still confused after reading many pages).

***Where do I insert custom code? For example, how do I know if it should go in the 'edit/style' area of header, pages, etc. or 'add html/css inserts'? Could someone explain this please.

***How do I change things like (I don't expect exact answers for all of these, but could you let me know if it requires custom coding and how to figure out where to put it?):

-my page menu bar has rounded edges, and I want them to be sharp corners. Do I need custom code for this - where would I put it?

-getting rid of bars that are hugging the top and bottom of my header image (and no, I don't have 'bar1 or bar2' in the header area on - when I downloaded the theme, they were already there. I spent like 2 hours trying to find out where it was in the options but I think it's in the code).

Those are just a handful of the questions I have, but I don't want to overburden anyone with them. Mostly, I'm just REALLY confused about where the code goes, and how to know for sure you are putting it in the right place. Thanks in advance.