How to use this phrase"how come",what kind of Situationcan I say it? please give me some examples to explain it .thank you!

First notebook entry: Introducing myself Hello,I m Sophie from BaoTou ,China.I m a collage student major in Chemical.I was kind of interested in English when I was preparing for Colllage English Test.I realized that English was such a beautiful and Rhythm language,so I’m desirous to express myself in native English ,I hope friendly people here could help me to improve Basic English ability on reading, listening and speaking.And also I would try my best to help someone who like to learn Chinese.