It's kinda tricky to tell u tips if we don't know you're driving style, but I'll pop a few tuning pointers ur way...
Toe-in in the rear increases oversteer, as does toe-out in the front...they may help turn-in ability...
Softening the front springs a bit may help if it's plowing into the turn, as will firming up the rear to increase oversteer...
You may also try to alter your brake balance to be more to the rear so the front tires will have more traction to turn...
Add some downforce may also help...
There's just some novice tuning pointers...they may or may not help, depending on the car and driving style...
Hope it helps!
EDIT: Also, if you're using the wheel, there's no force feedback in GT5 for tire slip, so be sure that you aren't over-turning the steering wheel...If you're turning and you see the front tire temp turn red, you're overturning, and this causes oversteer.