Corrections in blue.  Stylistics in blue.

My Old Friend :)

Hi. Hi all, again. )  How is it going? [1] 
Today was a nice summer's day.  It was a bit overcast, and it rained.  (But it wasn't terrible; the rain has passed quickly.
Today I met and talked with a [2] girlfriend.  Her name is Diana.  She and I are [3] classmates and best friends. From the very first class, we were inseparable. But after completing the fifth grade, we became separated, socialized with different people, and each of us had a different best friend. For a while we stopped communicating altogether. We communicated only in English and French, subjects we studied together in school.  So, today at 12:00 I met with her [3].  We walked. we talked about summer, and about a school that does not want to come back. [4]  In fact, we chatted about everything. Our communication was restored, and she is still my best friend, but not so close as Nastia.  :)

[1]  I think you wanted to translate "Как дела?"  If so, "How is it going" is a good way.

[2]  I don't know why you wrote "his" (его).  I think you meant you met with a girlfriend of yours.  

[3]  In Russian, you have a construction "мы с (кем-нибудь).  We don't use that construction in English.

[4]  I don't understand the phrase, "about a school that does not want to come back."  If you write it in English in other words, or write it in Russian, I'll try to help.

You did a good job.