Secure my router behind firewall as if it is compromised already.

Secure my router behind firewall as if it is compromised already. I have basic knowledge of networks, but far from being able to secure the router properly. I don't use my computers for purchases, but I'm setting up linux (which I don't know much about) and trying to install secure communication apps, but ther seems to be some connectivity issues, and one of the computers with hardware issues seems to have compromised hard drive. I don't know if it's a virus, but