Today was a very tireding day.

I am reading about protein.

Protein is very important for the people.

I prefere the natural protein like a eggs.

Bodybuildingers usinge protein in big quantities but for the rest of people is not necessary.

In the supermarket there exist protein products in abundance, it's so crazy.

It's is a contraction of "it is" in that context it needs an apostrophe.

Bodybuildingers usinge protein in biglarge quantities but for the rest of people ius that's not necessary.

We don't use "big" to describe the amount of something, it's used to describe physical size or importance (ie. "a big step forward," "a big moment"). Usually we say "large" but "vast" and "enormous" etc are okay too
"unnecessary" can also replace "not necessary"

In the supermarket exists there are protein products in abundance, it's so crazy

"there exist" is grammatically correct but sounds very stilted

Today was a very tireding day.

I've been reading about protein.

Protein is very important for the people.

I prefere the natural protein, like afrom eggs.

Bodybuildingers usinge protein in biglarge quantities, but for the rest of the people, that is not necessary.

For things which are general facts, simple present tense works better than the progressive. So, "bodybuilders use protein" instead of "bodybuilders are using protein."

"Big" is usually used for physical size (height, width, and volume), but feels awkward when used for numbers and quantities. "Large" might work better here.

Here, instead of "the people", I might say "us": "...but for the rest of us..." This might sound more natural.

In the supermarket exist, there's so many protein products in abundance, it's so crazy.

Today was a very tireding day.

I was reading about protein.

Protein is very important for the people.

I prefere the natural protein, like a eggs.

Bodybuildings using protein in big quantities but for the rest of people is noers eat large quantities of protein, but for most people that isn't necessary.

In the supermarket exist protein products in abundance, its soThere are so many protein products in supermarkets that it's crazy.

Today was a very tireding day.

I was reading about protein.

Or you could also say " I read about protein"

Protein is very important for the people.

I prefere thea natural protein, like a eggs.

Bodybuildings usingers consume protein in big (large) quantities, but for the rest ofmost people it is not necessary.

In the supermarket exist protein products in abundance, its so(grocery store), there are so many protein products it's crazy.