On second thought, I think 开口 is best translated as ''to start a conversation" or "to begin a conversation."  We also often say ''to strike up a conversation."  Any of these is OK  Just don't use "'open my mouth" in this sense.

like to exercise my speaking english5

I want to exercise practice my speaking spoken english ,  But i don't know how to open my mouth start to talk  ,and don't know how to say speak. the topic ,the job,life,culture ,economic?which is the best?  Which of these topics is best:  my job, my life, the country's culture, or the economy?  (Or:  As to the topic, which is best?  Jobs, life, culture, economy?)

Although 开口 can mean "'begin to speak" in Chinese, in English it means only to physically open one's mouth.