All of my corrections are highlighted below.
Hi all, my name is Sefa. I'm from Turkey, and I'm 27 years old. I have finished college. My first college section is Marble Technician. My other sections are Foreign Trade and Business Administration. I'm working in a Foreign Trade Company where I work as an Export Manager, but I'm a new. I want to improve my English so I can make new friends and find new customers. We are producing Natural Stone and we manufacture and export. My English is not there yet, so I'm asking for your help. Thank you.
Regards,
Sefa
First, I would combine some of the sentences by using conjunctions. Also, I'm not sure if you mean degree or major for the word section, so I didn't do too much with that sentence. Also, remember to put spaces between commas and words. Also, watch when you are using past and present tense.