American english or british

good evening, I'm learning english in a private school, I am now on level 4, the teacher of English told us that we must choose an american or british pronunciation when we speak  and not a accent mixed, for me it's a little difficult because listening websites on the net that help us to understand the conversations are mixed so we learn words with different pronunciation therefore vocabulary with pronunciation mixed,for ex concerning my case i love amercian accent but i can't learn it pure because i accross from always with british pronounciation; please can you help me with advice for solve this problem, thank you all.

Hello Amilla85, I wish you the best of success and progress in learning the English language. 

Actually, there are so many good listening websites that don't use mixed accents. Since you prefer the American English, try this one for example. You'll learn so many new words. You can also download podcasts galore & for free!

Enjoy

thanks a lot.