The prefix key (Ctrl+B by default) activates Tmux commands. Press it, release, then press the command. For example, Ctrl+B then c creates a new window. Change the prefix in ~/.tmux.conf with set-option -g prefix C-a.

Common key bindings (prefix = Ctrl+B):

Sessions:

prefix d            detach session
prefix s            choose session
prefix (            previous session
prefix )            next session
prefix $            rename session

Windows:

prefix c            create window
prefix n            next window
prefix p            previous window
prefix l            last window
prefix w            choose window
prefix ,            rename window
prefix &            kill window

Panes:

prefix "            split vertically (top/bottom)
prefix %            split horizontally (left/right)
prefix o            next pane
prefix ;            last pane
prefix x            kill pane
prefix z            toggle zoom (fullscreen pane)
prefix q            show pane numbers
prefix {            swap with previous pane
prefix }            swap with next pane

View all bindings: tmux list-keys. Create custom bindings in ~/.tmux.conf:

bind-key -n C-l send-keys -R 'clear' Enter  # Ctrl+L to clear screen (no prefix needed)

Use -n flag for bindings that don't need the prefix.