@Shinhyung,
Welcome to the Philforum!
You will find many different discussions here, and reading some of them will point you toward further reading, especially in the sections devoted to particular philosophers. Please feel free to post in any forum if you feel comfortable doing so. A good place to ask questions might be the "Phil.101" Forum (see the pinned topic for guidance about the kinds of questions).
I do not know about the availability of English language books where you live, but there are many philosophical texts available on the internet. John Stuart Mill's "On Liberty" and "Utilitarianism" are classic statements of important philosophical positions, well-written essays, and aimed at an educated reader (instead of other philosophers).
You might also want to try reading Descartes with a view to at least understanding his method, if not some of his conclusions. His Discourse on Method is very approachable.
The Enchiridion by Epictetus will introduce you to Stoicism; it is a brief introductory "manual" for his students, and another philosophical "classic."
Also on-line are very good philosophical dictionaries which may be of help in your reading; the Stanford Encyclopedia of Philosophy, although written for more advanced students, is also a good place to find information (preferable to Wikipedia).
Nothing beats reading the philosophers themselves at any level, even if you don't understand some of what you read. I suggest avoiding secondary sources or books "about" philosophy or philosophers; either these are aimed at readers with a strong background in philosophy/ or are extremely simplified discussions aimed at little old ladies who meet once and month to discuss "ideas."
Regards, and again, welcome.
John