Focusing for 50 minutes (most lectures are approx 50 minutes’ long) is difficult at first but it is good to train yourself to be able to concentrate and note-taking is a really useful skill.
Please remember that the lecturer is not there to entertain you. Most will try to make things interesting and may well be humorous at times – it’s one way of keeping the audience engaged and therefore open to learning. If you find your lecturer interesting and at times entertaining then that’s a bonus. Your lecturer is there to teach, not to entertain.