All ports seem to be closed

All of my ports seem to be closed according to various port testing sites. Is this a simple fix, or am I doing something seriously wrong somehow?

So, you open them... check your router's config

Yes, go into your router and open your ports.

>Is this a simple fix

Most of your ports should either appear as closed to the outside, or invisible completely.  You should only forward traffic from specific ports to specific end points (a computer, a network appliance) as needed.