Open your ~/.bash_profile (or ~/.bashrc) and add a line:

export PS1="[\u@\h:\w]\$ "

Possible options (that make sense) are \t (time 24h), \t\@ (12h[AM/PM]), \H full hostname (\h is hostname up to first dot) and \W for current dir name. You need to login again or just reload your prefs manually:

shell> . ~/.bash_profile