The i3-wm does not come with a keybinding to lock the screen or a preconfigured auto lockscreen. This gist describes how to setup both using i3lock and xautolock. i3lock is a minimalistic lockscreen and xautolock monitors mouse and keyboard activities to automatically lock the screen after a certain time of beiing inactive.
First get the tools if neccessary.
E.g. sudo apt install i3lock xautolock.
To setup the keybinding Ctrl+Alt+l (last one is a lowercase L) to lock the screen append the following lines to the i3 configuration file located at ~/.config/i3/config.
# keybinding to lock screen