Hi !!!

Hi guys, I´m Noelia and I I'm new around here, so I speak a little about me I'll tell you a little about myself. I´m 20 years old, but tomorrow is my birthday. I'll go all day on the beaches of Tarifa with my two friends. (Although not really incorrect, a more natural way to say the previous sentence would be "Tomorrow I plan to spend the day with two of my friends at Tarfa Beach.") I can do this because I'm on vacation and I live in Cádiz; however, on in September I have to go back to Granada to continue with the my law studies. We are in contact!. (I'm not quite sure what you're trying to say here. Perhaps you want to say "We'll stay in touch.")

Have a great birthday!

(introduction) Hi !!!

Hi Guys, I am Noelia and I  am new (around) here  in this website , so let me introduce myself in a few words. I am 20 years old, but tomorrow is my birthday.( I am going to be 21)  I will ( if it is sure: I am going to ) spend my (whole) day on the beaches of Tarifa with my  friends .(With two of my friends ) . I can  (easily) do this( it is possible for me )  because  now I am on vacation and I  am living  in Cádiz however on September I have to go back to Granada to continue with the law studies.( to follow my  studies) We are in contact!. ( be in touch / keep contact ) 

Notes : 1. instead of live : living is better because now you are living there but you are going to move, so it is just temporary.

2. if an occasion, event , any  other fact is sure you should use the  present continuous form or the structure of "going to " ( the first one is the most certain  , so that is a fact )

(Your introduction is quite good:) !  , Most of the corrections are suggessions ) In case of any questions,  do not hesitate to contact  me !  :) 

Happy Birthday :) !

Cheers,