Assuming you are talking a Oday Outlaw there are many O'day owners groups on the web. Generally speaking everything you would want to do has already been done by someone else (unless you want to put a 600hp motor on it
This is a good start:
The outlaw is (IIRC) a 20ft or so, Cam's post above highlights the basics.
The first thing you will need to do is decide exactly what you will be using the boat for. If it's simply day sailing you need no electric system at all - a handheld VHF and GPS will do, and save you a bunch of money.
Everything after that gets more complicated. If you want to use the boat as a overnighter you'll need anchor lights, running lights, steaming lights, interior lights etc., and you need to be sure the meet the requirements. No fear, google it up and the information is available.
Once you've figured what you want to do with the boat there are plenty of folks here that can help you lay out a system that will meet your needs.
A simple system:
1 deep cycle battery - $80 http://www.defender.com/product.jsp?path=-1|328|51495|306219&id=777510 (cheap and 'okay' for a weekend of small boat use).
1 4 circuit panel - $100 http://www.redsunfineart.com/files/p...gs-no-ham2.jpg
Switches, boards (plywood) wire and battery boxes, $100.
Lights, interior (2 or 3) $30-40
Masthead anchor light $40 http://www.defender.com/product.jsp?path=-1|17|295769|320396|320404&id=131699
It adds up quick
always multiply estimates for costs by 3 and time by 4