Welcome to the forums.
The only way to improve is to practice and practice and practice. Vocabulary is learned through reading and listening. Read and listen as much as possible. Have a good dictionary and a notebook - when you meet a word you do not know, look it up and then use it.
You can always try out your English writing on us.
Basic rules in writing:
Each sentence starts with a capital letter.
Proper names [personal names/names of the months and days/company names/names of languages and countries] start with a capital letter.
One space follows a punctuation mark.
Learn to punctuate: http://www.usingenglish.com/glossary/punctuation.html