Well, this is my first piece of writing and I don't know what to write about,

So, I thint I'll write about myself...

My name is karla, i'm 25 years old, i'm mexican, I live in Mexico City and hjajaja I feel like I'm in English class you know ... Introduce yourself and every stupid thing, so writing about myself is really stressfull because i want it to be nice... So that's all for the moment, when i have more to show i'll writte a little more.

Peace & love, Gorgeous

(Thanks for calling me Gorgeous.)

Not bad. Here are the main points:
Always capitalize "I" and proper names including your own name and names of nationalities and languages, such as English.

"Know" is a verb, "now" /nau/ is an adverb with a completely unrelated meanding.

When the subject and object are the same, always use the reflexive "myself," "yourself," etc.

My fFirst writtenWriting

Well, this is my firtsfirst writtenwriting and i don't now about what to write,. (or "...")

So, i thintthink I'll writte about memyself...

My name is kKarla, iI'm 25 years old, iI'm mexican, I live in Mexico City and hjajaja(we use "hahaha") i'mI feel like I'm talking to you in my eEnglish clasesclass you now ... Introduce yourself and every stupid things about yourself,. sSo writewriting about me is really stressfull because iI want to be nice... So it'sthat's all for the moment, and when iI have more to show iI'll writte a little more.

Peace & Love Gorgeous

Great job!

Phil's corrections are great and a lot more authoritative, but here are some more thoughts.

My first written writing

Well, this is my firtsfirst written {"my first writing" OR "my first written note"} and I don't know what to write about.about what write,

So, I think I'll write about me...

My name is Karla, I'm 25 years old, I'm mexican, I live in Mexico City and hjajaja i'm I feel like I'm in my English classes you know? ... Introduce yourself and every stupid things {wasn't sure about what this part meant}, so writing about myself is really stressful because I want be nice. So it's that's all for the moment {better: that's all for now}.  When I have more to show, I'll write a little more.

Peace & Love Gorgeous

-----

Nice job on your first note!  You already understand some of the crazy grammar in English so keep it up!  You'll be great in no time  :)