Hello,
Before writing anything it is advisable to ask questions such as

• Purpose of writing: To inform people, criticize, appreciate, motivate someone. With the purpose in mind you will be sure about the tone and the content of the whole writeup
• Target Audience: You should know who your readers would be and write accordingly. For example writing in layman language if readers are general public or if readers are scholars then giving technical details
• If you have enough knowledge to write about the subject

Writing is a very dynamic subject, no hard and fazt rules are to followed, experiment and try to find your own writing style. The above questions might help at the starting.

Good luck