How do I find out what service is using a certain port?

netstat -b in the command prompt will also work. Sysinternals TCPView is basically a prettier GUI version, netstat is a tool that comes with Windows.

Sample output:

Active Connections

  Proto  Local Address          Foreign Address        State           PID
  TCP    john:2817              localhost:2818         ESTABLISHED     972
  [firefox.exe]

  TCP    john:2818              localhost:2817         ESTABLISHED     972
  [firefox.exe]

  TCP    john:2821              localhost:2822         ESTABLISHED     972
  [firefox.exe]

  TCP    john:2822              localhost:2821         ESTABLISHED     972
  [firefox.exe]

  TCP    john:3177    peak-colo-196-219.peak.org:http  ESTABLISHED     972
  [firefox.exe]

  TCP    john:3182    peak-colo-196-219.peak.org:http  ESTABLISHED     972
  [firefox.exe]

  TCP    john:2879              67.69.247.70:http      CLOSE_WAIT      972
  [firefox.exe]

  TCP    john:2880              67.69.247.70:http      CLOSE_WAIT      972
  [firefox.exe]

  TCP    john:2881              67.69.247.70:http      CLOSE_WAIT      972
  [firefox.exe]

  TCP    john:2882              67.69.247.70:http      CLOSE_WAIT      972
  [firefox.exe]

  TCP    john:2883              67.69.247.70:http      CLOSE_WAIT      972
  [firefox.exe]

  TCP    john:2884              67.69.247.70:http      CLOSE_WAIT      972
  [firefox.exe]

Sysinternals TCPView will show you TCP/UDP ports that are in use and the processes that are using them.

netstat -an will show all the ports which are currently open with their address in numerical form.
To find info about a particular port use via Power Shell's prompt as Administrator: netstat -an | Select-String 10000.