Press the Settings button. Choose the Hotkeys pane from the left hand side.
For each of the listed functions, choose a hotkey to assign by pressing the + icon on the right side of the window alongside the desired function.
Hotkeys can be assigned to starting, stopping, pausing recording, performing a transition, switching to a scene, showing and hiding sources like display capture, etc.
Googling for "obs configure hotkeys" turns up a number of guides and youtube videos.