Look out for prepaid booth of Delhi Police for Auto/Taxi and use the services.
The best alternative (if you have less/medium luggage), come out of Delhi station and ask for MEtro station. take a metro(towrds Central Sectt/Gurgaon side) to NEw Delhi Station which will leave you near the Car Parking (side of PF 16). TN Express departs generally from pf 6 but some times it changes to 3 also. Pl enquire with the porter on the day of journey for correct platform no.
If you have a AC...
more... class ticket, Coaches are towards engine side (Towards nizamuddin) and SL class - S1 -to PC/S6 are easier accessing towards engine side and S7 onwards towards the rear side. (towrds Paharganj bridge - red colour visible from platform)/Parcel office side
have a pleasant and safe journey.