There is only one way to improve your writing, you should write a lot

The first step to improve your writing is understanding your writing goals.

For example, I typically write letters for e-Bay or Amazon sellers, sometime I report about bugs in software applications. And it's (while that's technically correct, most native English speakers would perfer "that's all" in this context) that's all. I've never sent emails in English for working reasons.

I dread writing letters for working purposes. My grammar isn't good, sometimes I don't know the certain meaning of words, and sometimes I mix prepositions or and articles. So I feel I'm not ready to use writing skills.

For (the purpose of) writing better, faster, and in a more official way, I've decided to try pass the IELTS General exam. There are two types of writing tasks: a letter about some official reason affair and an essay about a general themes. Therefore I need to practice this these kinds of writing skills. I found a lot of typical IELTS essay themes and decided to write one essay or one letter per day. But I don't know who will correct me...

Maybe you can recommend me something? I will be appreciate your advice.
Thanks for your attention and don't hesitate to correct this writing or write a comment, please do it, I need it a lot.

Your English is great! I'd suggest for you to write something everyday and maybe try reading a book with a more advanced vocabulary just to brush up on your skills. Good luck with the exam!