Almost 10 years ago I and my friend Lena managed to get the free vouchers for student summer camp.

I would say "me and my friend Lena," although some may say "my friend Lena and I" is the only correct way.

This summer camp is located on the Black Sea coast.

You could also say "on the coast of the Black Sea."

I was nineteen, she was twenty, and we haved just finished our exams and summer practices.

Not toI should mention it was the first time she saw a'd seen the sea, while I used to be at seacome to the seaside with my parents only.

We usually say "the sea" or "the ocean" instead of "a sea" even though "a sea" is technically more accurate.

"Not to mention" doesn't quite fit here, because there's no a previous point for it to expand on.

"Seaside" works better as a destination than "the sea" because your hotel is next to the sea, not in it.

You might be thinking thatDid our days turned into a constant rave?

Or "You might be thinking that our days turning into a constant rave." "You might be thinking..." is a statement, so a question mark and an answer is wrong.

We had a very calm and decent life in an old woody cottage, with a balcony.

Beautiful! Sounds like a line from a storybook.

My day was scheduled in such athis way: I got up really early for a yoga class, then I had a pretty long swim; breakfast was served in a student canteen, so I picked up some food for always sleeping Lena, and waoked her.

At the noon we were walking, hiking, mtaking gigabytes of photos, then we swam in the sea 'till sunset.

'til is an abreviation of "until" so you need an apostrophe to show letters are missing. "Till" is a different word.

So I needed a good sleep.

Maybe connect this to the previous sentence: "You can guess that I was always exhausted by the evening, so I needed a good sleep."

We both were desperate for some messy adventure.

"Messy" is an interesting word here. It's not one I've seen used to describe an adventure before, and I don't immediately know what you mean, but I like it.

- whispered Lena-, "but non-alcoholic"."

No dashes for dialogue, just use commas for pauses. And punctuation should usually go inside the end quote.

We both haved seen bars in the films only.

"We had both only seen bars in films" sounds a bit more natural.

That is where I’ve seen saw how to behave in such a places, and anwhere I got the idea that girls should order fancy cocktails.

“Two… shakes….

A new person is talking, so this should go on a new line.

”- I blurted out.

Haha, I like bit of dialogue. Lots of personality.

Even athe hard-boiled bartender was surprised.

"hard-boiled" is also an interesting description. It's not a commonly used way of describing a person, but again, I like it.

"We don’t serve milkshakes ...." - he held out -," he said, "but I’ll do it for you".

I'm not sure what you were going for with "he held out."

Be careful not to overuse elipses (...). The break with "he said" serves a similar purpose, and doesn't seem overdramatic.

So we hadgot our milkshakes at almost 2 am, drank them and we called it a night.

"Had" and "drank" mean the same thing here, so it's odd to use them both. "Got" here would mean "ordered."

The next night she woke me again: "I say to'm telling you, get up!".

"I say to you" is a bit formal and awkward.

"Order smthomething alcohol-free, but not milkshakes!"

Abbreviations like "smth" are only used when you're writing in a hurry or in restricted space. In a story like this, we'd only use abbreviations that mimic speech (like "you're" and "we'd"). "Smth" isn't really pronouncable, so it's just making things more difficult for the reader.

She was too shy to talk to someanybody.

So I started to examine the wine list or whatever it was.

Can be connected to the previous sentence: "... talk to anybody, so I started..."

"Mojito" - I found a familiar word.

A dash could actually work here, although you'd want to use an em-dash (—) rather than a hiphen (-). For some reason, these are really hard to type, so you can use a double dash to approximate it (--)

I'd had mojitos at the cafes as the alcohol-free type.

And I had no clue what is the original recipe. was.

Even before we got itthey reached us, we fsmelt anthe alcohol smell.

"Yes ...

I had it at the café ...".

"Are you SURE it is alcohol-free ???"

Multiple question marks are only used in casual conversations, you wouldn't usually write like that in a story.

- Lena exhalesd.

Keep it past tense.

"Absolutely sure ...," -I whispered I.

You can say "whispered Lena" but "whispered I" is too old fashioned.

So we had for half of our drinks when Lena finally found out from the bartender it was rum-based.

And I have had no step at the bar since that tim've never stepped into a bar since.