I just want to know if this paragraph is right, and it doesn't contain grammatical errors. Thank you for the answer!

"I brought with me this biscuit, banana and chocolate, which characterize my current eating habbit. In the exam period students think that they don't have time to waist. They avoid cooking and they chose the easiest way, I mean eating in the nearest fast food. I think healthy eating can improve the “biochemical prosess” of learning. So I try to compensate, (to counterweight?) the unhealthy fast foods with fruits like bana apple and orange. The biscuit is for breakfast. And chocolate is my special brain booster. So I’d like to give you this little present with which, I wish you a healthy and merry Christmas."

I see comma errors, subject-verb mismatch, spelling/word choice errors, at least one run-on, misuse of punctuation, and maybe others.

Can you please help me to correct them?

You'll have to isolate each sentence and tackle it separately. If you try to do the entire paragraph at once, you'll get lost.

I brought with me this biscuit, banana and chocolate, which characterize my current eating habbit.

Comma errors cause confusion. Did you bring three things with you? If so, you need another comma. Do you see where?

What is the subject of the verb "characterize"? Singular or plural? Does it match the verb (s or pl)?

Are you using a word processor with spell check? If not, why not?

No spell-checker will pick up on word choice errors. You'll need to find and fix those yourself. For example, using "there" when you should use "their" is a word choice error, not a spelling error.

The comma rules: http://grammar.ccc.commnet.edu/grammar/commas.htm

Subject-verb agreement: http://grammar.ccc.commnet.edu/grammar/sv_agr.htm

Run-ons: http://grammar.ccc.commnet.edu/grammar/runons.htm

Fix this one sentence at a time! Start with this first one.

The paragraph you provided contains a few grammatical errors that need to be addressed. Here is the corrected version:

"I brought with me this biscuit, banana, and chocolate, which characterize my current eating habit. During the exam period, students think that they don't have time to waste. They avoid cooking and choose the easiest way, which means eating at the nearest fast food establishment. I believe that healthy eating can improve the biochemical process of learning. So, I try to compensate or counterweight the unhealthy fast foods with fruits like banana, apple, and orange. The biscuit is for breakfast, and chocolate is my special brain booster. Therefore, I'd like to give you this little present with which I wish you a healthy and merry Christmas."

To ensure that a paragraph is grammatically correct, it is helpful to proofread the text and check for any spelling, punctuation, or grammar mistakes. You can also use grammar-checking tools such as Grammarly or Microsoft Word's spelling and grammar checker to identify errors. Additionally, reading the paragraph aloud can help you catch any awkward or incorrect wording.