how can i write a coposition (opinion on a given subject )
Can you be a little more specific?
Here is a website that may help you:
Guide to Grammar and Writing
Hotlist: Composition & Writing -this has many links to writing websites.
Basically, you need to make a statement about what you believe, and then you need to support it using personal experiences, facts, statistics,...whatever is appropriate to the type of composition you need to do.