Are you on a wheel or controller? It took me less than a day to get use to drifting in GT5 with the right tune. To be honest all that we can give you is advice and more than half of the stuff is you practicing. I would recommend the Nissan Silvia S13; you won't be able to catch up with faster cars but it's a good starter car. In my opinion you learn more when drifting with low HP cars and understand how the car performs.
Basic Settings:
Comfort Hards
Stiff the front and back (Including Dampers)
Not a lot of camber and a little toe in.
LSD: 2 way LSD - 5/60/60 = When driving straight the wheel acts like a open diff meaning inside and outside tyre spin at different rate and when mid drift both inside and outside tyre spin at same speed.
Lock/Wielded Diffs - 60/60/60 = Inside and outside wheel spin at same rate meaning would be slippery and kinda hard to get control of on straights but corners it's amazing.
I use Lock Diffs because it's easier for me.