[CentOS] Cents OS from Bash Shell

I want to know the right command to type from a bash shell so that i can
   1) Check the version of my cent os
   2)  Check all the open ports (tcp and udp) on my machine
   3)  Open a specific port say port 3306 so that a  telnet request from a
remote machiene can be accepted
  4)  Disable the effect of 3 above in case I want to

Thanks in anticiaption of your kind responses to the questions above.