Idiomatic expressions are VERY tricky to master in any language. As a beginner it is safer to avoid them or to focus on a few with which you can become comfortable. Check with native speakers about which ones to use. The incorrect use of an idiom or the overuse of an idiom is an immediate flag (indicator) that the speaker is foreign and sometimes can lead to embarrassing situations or misunderstandings. The misuse of idioms can even cause unintended offense so be very careful with them.

It depends on your job. As a teacher I use some idiomatic expressions in conversation with students. I use a lot of different ones with my friends.

"The more idiomatic expressions you use, the better speaker you are" ...hmm, no not really. How often do you apply idioms in Turkish? More importantly, WHEN? The real indicator is knowing when to use those idioms. Here is some food for thought: the moment you open your mouth and the (native English-speaking) listener realises you are not a native speaker, chances are that the English speaker will avoid using idioms with you (so as not to confuse you). It really depends on who you are talking to; though in general, you should be able to handle idioms quite well anyway.

I am going to agree with rajalala that idioms are very tricky. As to your thought about "the more idiomatic expressions you use, the better speaker you are", there could certainly be some truth to it. If you can understand and use idioms in context then you certainly have a better grasp at casual, conversational English. I would say that I use and hear idiomatic expressions every day.