Ok, the ticket with the best connection is an IC train, so you can't use the VGN Tages Karte ticket for that train. So buy on line at www.bahn.com. You can see the schedule on that site as well.
You can get a fully flexible ticket from Nuremberg to Rothenburg ob der Tauber for 20 euro. You can use any trains. You can print the ticket out at home and bring it with you. According to the schedule with the fastest time, you only have 13 minutes at Nuremberg station, so it is good to have your ticket in hand. You will arrive on track 17 and leave on track 7, at least as the schedule is now. Tickets are available for purchase 3 months before you travel date.
Edit- Here is the schedule from the Deutsche Bahn website. Assume you are traveling solo.
Praha hl.n.  We, 13.04.16  dep  10:37    SC 512 SuperCity Direction: Frantiskovy Lazne 
 Subject to compulsory reservation , Bicycles conveyed - subject to reservation , International supplement , Bordbistro , space for wheelchairs , power sockets for laptop , handy/quiet zones , parent-and-children compartment , Wifi available
Cheb  We, 13.04.16  arr  13:14     
Transfer time 24 min.
Cheb  We, 13.04.16  dep  13:38    RE 5286 Regional-Express Direction: Nürnberg Hbf 
 Number of bicycles conveyed limited , air conditioning
Nürnberg Hbf  We, 13.04.16  arr  15:22 17  
Transfer time 13 min.
Nürnberg Hbf  We, 13.04.16  dep  15:35  7  IC 2064 Intercity Direction: Karlsruhe Hbf 
 Bicycles conveyed - subject to reservation , Number of bicycles conveyed limited
Ansbach  We, 13.04.16  arr  16:06  2  
Transfer time 4 min.
Ansbach  We, 13.04.16  dep  16:10 3  RB 58122 Regionalbahn Direction: Würzburg Hbf 
 Number of bicycles conveyed limited , vehicle-mounted accessaid , air conditioning
Steinach(b Rothenburg ob der Tauber)  We, 13.04.16  arr  16:31 2  
Transfer time 4 min.
Steinach(b Rothenburg ob der Tauber)  We, 13.04.16  dep  16:35 5  RB 58903 Regionalbahn Direction: Rothenburg ob der Tauber 
 Number of bicycles conveyed limited , 2nd class only , vehicle-mounted accessaid, notification: 01806-512512 * , (*20 ct/call from German landline, mobile phone max. 60 ct/call)
Rothenburg ob der Tauber  We, 13.04.16  arr  16:49 1