To learn proper pronunciation, you must first hear proper pronunciation. If your teacher is not speaking natural English, you must find material that does speak natural English. The very best way would be to find a native English speaker to talk to. Unfortunately, if you cannot find a native English speaker, the only other way to learn proper pronunciation will take a lot of extra work. You will need to find a recording of a native speaker. This part is easy, either find an English TV show or movie, or you can go online to youtube.com and choose from millions of amateur videos with native English speakers for free.
Listening is the first step. The second step is to try to reproduce the sounds. Find a sentence spoken in native English, listen to it, pause the recording, and then try to repeat it. The key thing to listen for is rhythm. English is spoken with stresses and unstresses that, taken together, will form a noticeable rhythm, like in music. After you practice, try to repeat the recording, and speak along with the recording. Try to match your rhythm to the native speaker. It may help if you record your own attempts, so that you can replay them and compare to the native speaker.
I hope this helps!