I takeook a class abouton diabetes today.

The way you phrased it is still comprehensible. However, "took" (the past-tense of "take") is used here because the class already ended.
Additionally, it sounds more natural if you use "on" instead of "about."

I've learn so much about the types of dieabetes and the mechanisms how they happened in human body.

A few spelling errors there, but nothing major. Since there are different kinds of diabetes, there ought to be different mechanisms for each.

I also shared some of my views with my classmates, and we haved a hot discussion (in this class).

Past/present/future tenses in English can be frustrating! Present tense would be 100% correct if you were IN THE MIDDLE of class right now.
In addition, since the topic of this journal entry is about this particular class, you do not need to repeat the last 3 words. Leaving them in may be a bit redundant.

Finally, our teacher praised us for our enthusiasm infor learning.

I'm not too sure why we use "enthusiasm FOR" rather than "enthusiasm IN", but it sounds more natural this way. Once again, you are still 100% comprehensible, but it sounds slightly off. Additionally, you may add a coma between "finally" and "our" to help with pacing.

Anyway, here are my closing remarks:
I really appreciate the effort that you put into this journal entry! English grammar can be quite inconsistent and frustrating at times, but I really hope you keep at it!