I'm asuming you have an R... But if you runa striaght pipe it will cause CEl and be really loud.
the better method of opening up your exhaust would be to purchase a built one. There are many different kinds just have to see which you like and can afford.
What I would do to keep it affordable, quiet and with a decent power gain is OEM JDM 4-1 with a carsound cat and a Mugen twin loop rear section.
There are just so many different options for exhausts you should do some research and listen to fellow R owners.
Hope my blabing has helped.