Firewalls

I recently had an issue with my internet. Apparently, and I know not much, my internet had been accessed and many people from many places were using my internet. I was told I needed an internet firewall, not to be confused with firewall on my computer. So I purchased 1 year contract with Netgear, out of fear. I have a Mac Air, which has a firewall on it. If I’ve made sense at all, I would appreciate your opinion.