Reading is one of the secret weapons for improving grammar skills. As you read, you will notice different grammar rules and naturally start using these correct grammar rules in your writing.
In French, there are two kinds of articles, definite (Le/La/L'/Les) and indefinite (un/une/des).
Examples: C'est un stylo. C'est le stylo de Flora.
It is useful to have a grammar book to refer to as you write. Most importantly, a grammar book will help you avoid grammar mistakes and understand the text.
Read more books, magazines and newspapers to expand your French vocabulary. You can also consider taking one of the many online French courses to build a wider range of skills.
The more time you spend writing using correct grammar rules and techniques, the faster your grammar skills will improve. Also, there are lots of great grammar quizzes so get some quizzes and use them to test your level.