i have a system.
you need a head unit, sub, amp, and speakers; probably a capacitors to save your alternator from going bad quickly as the accord has weak alternator.
For the head unit i would recommend getting a double din. meaning it takes the full space of the unit, like stock.
as far as everthing else goes, it will vary on your budget.
check out
Crutchfield: Car Audio, Home Theater, Speakers, LCD TV, Plasma TV as a good reference source.