The Basics
Commands
List tmux sessions: 
$ tmux ls
tip: 6 windows (created Thu ...) [80x24]
main: 10 windows (created Thu ...) [117x55] (attached)
2: 1 windows (created Sun ...) [117x55]
3: 4 windows (created Wed ...) [117x55]

Reattach to an existing session named "tip": 
$ tmux attach-session -t tip
If only one session is running, 'tmux a' is enough.

If you haven't named your sessions, 'tmux a -t 2' works.