Securely Overwriting HDD for Reformat

So, i'm really new at partitioning, and overwriting data before reformatting.  I was wondering ...

Am I going to fuck something up to the point to where I can't fuck up anymore?

And...

How should and what should I overwrite the data with (specific programs, boot disk types)?

Tips on partitioning and how -you- personally partition your HDDs would be nice too.

>>3
"Only Windows artificially limits the size of FAT filesystems"
There are reasons for it. Lurk moar.