As with everything else, the key is practice, in my opinion. Meaning, the more you practice, the faster you’ll improve.
To improve your general writing skills, it may be a good idea to find pen pals who you could write to regularly.
There are lots of websites on the Internet to help you find the right person.
This one seems quite practical as you can also select the age group you’d like to get in touch with:
You could also use some online help with grammar and spelling:
18 Powerful Websites to Improve Your Writing Skills in English
Use these Grammar and Spell Checkers to Instantly Improve Your Writing
6 Ways to Improve Your English Writing Skills Using Cool Websites
And if you need more formal, business English expressions, I suggest you have a look at this article:
Formal and Informal Email Phrases in Every Situation
7 Simple Steps to Improve Your Business Email Writing Skills in English/
I hope you’ll find the above tips useful! Let us know if we can help you with anything else. Also, feel free to share your on tips and ideas with us in the comment area below.