Homebrew PCBs

Hi all,

I was wondering it there is an introduction to basics of schematics 
and board design?

Things like what polarity is the square pad on an LED ?
Why 15 mil traces and not 12.5 mil ?
how to best isolate an AC rectifier/cap from the rest of the DC on 
the board ?  things like that.

are there 'standard' drill sizes that are common for all board 
houses ?

I have a friend who says all square pads on 2 pins are ground or 
negative. Is that rule of thumb or general industry practice ?

and it seems a 90 degree corner on a 15 mil trace, when etched can be 
undercut to less than 10 mil.

Instrumentation inputs may be put inside of a ground ring to as to 
keep all stray baord voltages away.

Seems I went to the monkey-see monkey-do school of board design.

Dave