Use the following chatcommand:
/combatlog
Upon enabling WoW will print the logfile name. On Windows this can usually be found in the following path:
C:\Program Files (x86)\World of Warcraft\[_retail_|_classic]\Logs\WoWCombatLog.txt
Note: WoW does only write files after the player logged out (character screen is enough). If the game is forceclosed such als Alt+F4
the log is getting lost.