Compiling a kernel is.. is... lol.. well in programming you create your source code, right ?? once you got the source you need to translate that to machine code. This translation is what is called 'to compile'
You dont really access
the kernel per say, not in this context. What you need to do first is to aquire the source code.
How to get it? well, there are many different ways. One is using apt-get, gentoo I think has it own way to install new aplications/packages.
Once you installed the souce files (can be found in /usr/src) you need to uncompress them first.
After you have done that, find a nice totorial/how-to, google for 'compile kernel gentoo' or something alike.
You need to configure the new to-be-compiled kernel... then compile.. then install.
What is the kernel you might ask? the kernel it like the most important pice of code in the operating sytem, is what 'command.com' was for MSDOS (not exactly.. but in lame terms)
 some typos.
When you configure the kernel-to-be-compiled, is when you need to select the specific hardware support you need.
Btw, me as the noob I am... took me 26 attempts before I got a nice kernel working with ALL my hardware. But most people are not this dumb.