Fistfighting is something that you can compete in by going to the local tavern. You can challenge the locals and win some orens or other trinkets.
There is no tavern in the Prologue but once you’ve given Triss the potion suggest not going to Leo’s funeral right away. Head to the first floor of the keep. Eskel may be walking along the corridor or in the Armory. Either way talk to him and accept his offer of doing some hand-to-hand training.
This quest is picked up in Fistfighting Chapter 1.
Back: The Witcher Walkthrough