Welcome to Linux.

Concerning your questions:

How to keep your system clean?

• Subscribe to the security mailing list/blog etc. of your Linux distribution and for software that you use
• Update your system whenever there are updates available and reboot your system after applying the updates
• Activate the firewall of your system and block all incoming traffic which was not initiated by your own system
• Only install software which is distributed with your operating system or which is well known and you download from the official distribution page (for the sake of an example: If you use Google Chrome download the package/binary for your Linux from Googles Chrome page)
• Use an adblocker for your browser like ublock origin

What not to do:

• Never install software found on the internet or a forum
• Never run arbitrary script from the internet in your shell

Doing the above and applying some common sense should be fairly secure. As a rule of thump: Less software is always better and well known software will usually be better scrutinized and more secure. (YMMV)

As a normal desktop user your chances of getting your system infected when applying above rules are very low and they are your best line of defense.

Securing a Linux system, especially in depth, fills books, and detecting an infection is another topic for specialists. One way to improve your chances of having a non infected system is using an immutable Linux distribution like Fedora Silverblue, which should in theory be more resistant to infections and which should in theory allow to detect infections easily.

Unless you have a reason to expect being personally targeted (in which case: good luck to you ;-)), the answer to infections and similar is having regular full backups of all your data, so in case of an infection you can wipe your computer and recover everything. You should have regular full backups anyway, in case your SSD fails, your computer gets stolen and similar threats to your data.