There's really no point attacking a WR unless you're certain you can take it, I only have a handful but I knew I could take them all.
Getting a setup right is crucial just don't expect to find one off the shelf that will knock 2 seconds off the WR, cause it ain't gonna happen. You need to get a good base and fine tweak it to suit your driving style, and ultimately to get the best out of the car on that particular track. So if you have no idea how to tweak setups I suggest you learn this bit first, or atleast the basics.
Next thing you have to do is start attacking the track, thing is no one can really tell you how to do this, you just gotta do it. Watch the current WR replay and see where you can improve, find out where he/she is slower and basically when you can stomp the power quicker then the previous person. Once you've found a few slow spots in the WR, go back to the track and practice those certain corners to get them perfect. These are the area's where you make the time, assuming the rest of the track is flawless and you cant improve on it which is never really the case.
And finally don't expect it to come right away, it probably wont cause you probably suck big time
PS : Split times are your worst enemy, I found that hitting SHIFT + F and just concentrating on my driving is when I'm fastest.
And always remember to take a dump before you attempt a WR, I hear once you get a WR for the first time your bowels go all funny and you start crapping uncontrollably.