I have a security concern to enquire about.
1. How can I find out if someone is 'trying to break' or has 'already breaked' in to my system, whether it is do harm or just to use my resources without my permission?
Nice question. Here's the answer: You need to know exactly how your system works, what process should run and which shouldn't. Make sure to monitor these settings with the appropriate tools and keep up-to-date about security concerns that are related to your hardware and software. If you don't know how to start it will take you a few years to reach that state. If you suspect that someone might have already broken into your system check the hints that made you think so and reinstall the system. Once an intruder has gotten root access it is extremely difficult to make sure that no backdoor is left to give access to the intruder. Sandy ~ What a useless answer. How about pointing this person in the right direction? For security concerns, start by installing tripwire, http://www.tripwire.org/ You can also check to see who is connected to your computer by typing "users" at your shell. See more than one root user logged in (unless you expect more) and you have problems. If you think your system has already been broken into, start by disconnecting the computer from the rest of the network. Check your logs under /var/logs/. No logs, then you may have been hacked. This, by no means, is an all conclusive list, however it is a good start. Good luck, ~James