How to close tab in tmux?

prefix+x: kill the current pane
prefix+&: kill the current window

Note the difference between pane and window. OP actually wants to kill a window. So prefix+& will be better suited especially when a window has multiple pane splits.

I think this is what you are looking for: http://www.dayid.org/os/notes/tm.html

Specifically this part:

kill the current pane: ^b x OR (logout/^D)  
close other panes except the current one: ^b !  

The formatting here is simple enough to understand (I would hope). 
^ means ctrl+, so ^x is ctrl+x. M- means meta (generally left-alt or escape)+,
so M-x is left-alt+x