Adjusting the tension of the thread on the sewing machine is an inexhaustible topic, it gives the most problems to beginners, I made a special video that deals with this topic, I recommend you watch it, English subtitles may contain errors, but if something is not clear, feel free to ask.
See how to adjust the thread tension on a sewing machine on our youtube channel or in the video below. To fully understand the topic you need to watch the entire video clip.