I was stupid and didn't add any security to my wirless network when I set it up. I guess I thought it would be quicker. Anyways, is there a way to just add it after you have everything set up? Or do I have to reinstall everything?
If you are on a home network/system with no services, no firewall is required or even recommended.
Firewalls have two uses:
1. Filtering ports, either by packet type or data content.
2. Segregating network traffic.
No need #1 If you are not running any services and your network topography doesn't call for #2, running a firewall is not only unneeded, but to do so would be a poor choice. By adding a firewall in this environment you actually decrease the security of your system by increasing its complexity (reduced assurances, and just another application that needs to be trusted and kept current) and surface area. For example a number of personal firewalls had/have issues of being broken my particularly agressive nmap scans.
It is important to only add counter measures in response to threats that justify them, in this instance, I don't see that being the case here.