what steps to perform to secure a fresh installion of Mint?

i need to know if there are any internal settings

The thing with Linux Mint is that it is configured to be ready to use on install. You need to update it, and that's all. You can use it. Of course, there are many things you can do. However, that takes a huge amount of time dedicated to learning about the system to do so. If you want to tinker with everything and learn as much as you can, I'd suggest Arch. Arch is a distro that's pre-compiled and has only the necessary things in it. It also has great documentation with a beginner's guide on installing it. But believe me, it is hard to learn and configure it properly above the very basic stuff.

Back to Mint, you don't have to do anything to make it secure. Although there exists a (very small) chance that you'll face some problems in terms of security on linux, it is much different than windows. You don't need an antivirus software. Malicious software can pretty much only be installed intentionally by the user. Past that, there's the problem with network security. I don't think you can do much more than set up a standard firewall, like you've already done, and let it run. There are vulnerabilities, but they aren't easy enough to fix or dangerous enough to have priority over anything else. You should probably just install what you want and start using it, and search non-Mint specific instructions online to learn more.