The Big Bang Theory is a comedy series. The series is about a group of people who work in the science-field. Watch the YouTube-video by Learn English With TV Series and do the exercise they provide. Turn on subtitles (ondertiteling) in the options. Watch the video three times.
First watch: Watch the scene with subtitles and try to understand as much as you can.
Second watch: Study all the most important fluency aspects of that scene - vocabulary, pronunciation and connected speech, cultural references like jokes and situational humor.
Third watch: Watch the scene again without subtitles and really test your understanding.