My friend

Sandro has been my friend for almost ten years now. I first met him at a (here use a because the reader doesnt know which concern it is. You use 'the' when the reader knows. If you mention it again in hte text you can use 'the' because we have already been introduced to it and know which conert) concert of one of my favorite guitar players. After the gig, we found ourselves at the same table in the club and our common friends then introduced me to him. He was friendly and sociable; we started to talk about music and realised that our tastes were the same.

Physically, Sandro looks younger than most other man in their forties. It’s not only because he does sports and has a muscular build, but his dress/fashion style is also modern and informal. His hairstyle is something similar to the mohawk. He prefers wearing casual clothes, and for concerts he always wears a pair of ripped jeans and a t-shirt with a cool inscription. I don’t think I’ve ever seen him in a classic suit.
As for his personality, Sandro’s most striking characteristic is his sense of humour, he travels a lot with his band and when we met for a cup of coffee he spoke/talked (past tense) about his adventures for ages . He’s very good-natured and if you need help he’s always here.

Sandro is not very patient and is easily annoyed when somebody doesn’t keep their promises. To tell you the truth, Sandro himself tends to be quite absent-minded; forgetting about birthdays and arranged meetings. I know he doesn’t want to offend anybody: it happens only because he’s always busy doing something. He spends a lot of time working and often stays in his studio recording new tunes till midnight.

All in all, he’s a good friend. Since the day we met we have grown very close and I’m very fond of him, even if he forgets about my birthday again. 

Very good! Keep up the good work. I hope my corrections help :)

My friend

Sandro has been my friend for almost ten years. I first met him Our first meet was at the concert of one of my favorite guitarplayers. After the gig we found ourselves each other at the same table in the club and our common friends introduced me to him. He was friendly and sociable; we started to talk about music and realised that our tastes were the same.
Physically, Sandro looks younger than most other man in their forties. It’s not only because he does sports and has muscular muscle building, but his style is also modern and informal. His hairstyle is something similar to the mohawk. He prefers wearing casual clothes, and for concerts his «uniform» is a pair of ripped jeans and a T’shirt with a cool inscription. I don’t think I’ve ever seen him in a classic suit.
As for his personality, Sandro’s most striking characteristic is his sense of humour, he travels a lot with his band and when we met for a cup of coffee he can talk about his adventures for ages . He’s very good-natured and if you need help he’ll always here be by your side.
Sandro is not very patient and easily annoyed when somebody doesn’t keep promises. To tell you the truth, Sandro himself tends to be quite absent-minded forgetting about birthdays and arranged meetings. I know he doesn’t want to offend somebody: it happens only because he’s always busy doing something. He spends a lot of time working and often stays in his studio recording new tunes till midnight.
All in all, he’s a good friend. Since the day we met we have grown very close and I’mvery fond of him, even if he forgets about my birthday again. 

My friend

Sandro has been my friend for almost ten years. I first met him at the concert of one of my favorite guitarplayers. After the gig we found ourselves at the same table in the club and our common friends introduced me to him. He was friendly and sociable; we started to talk about music and realised that our tastes were the same.
Physically, Sandro looks younger than most other man in their forties. It’s not only because he does sports and has a muscular build, but his style is also modern and informal. His hairstyle is something similar to a mohawk. He prefers wearing casual clothes, and for concerts his «uniform» is a pair of ripped jeans and a T’shirt with a cool inscription. I don’t think I’ve ever seen him in a classic suit.
As for his personality, Sandro’s most striking characteristic is his sense of humour. He travels a lot with his band and when we met for a cap of cofee he can talk about his adventures for ages. He’s very good-natured and if you need help he’s always here.
Sandro is not very patient and is easily annoyed when somebody doesn’t keep promises. To tell you the truth, Sandro himself tends to be quite absent-minded, forgetting about birthdays and arranged meetings. I know he doesn’t want to offend anybody - it happens only because he’s always busy doing something. He spends a lot of time working and often stays in his studio recording new tunes till midnight.
All in all, he’s a good friend. Since the day we met we have grown very close and I’m very fond of him, even if he forgets about my birthday again. 

[Excellent.  You don't have the problem with English articles (a, an, the) that many Russian speakers do.  Your English is quite advanced.]