Do you think it's a good idea to publish your writings on online forums?

Of course it will help you reach more readers.

First be sure what you want to do whit your writings. Sell or just share your thoughts with others?

Not all forums, specialized ones only where you can get feedback from other writers. Don't spread everything online and then when you want to compile them into a book perhaps they'd be old news. That's just my personal view though.

Well if you want it to reach to wide vairty of audience do it