i would like to know, is there any way, for example adding a shortcode, that will let user to put my plugin's widget anywhere he wants??? and how can i add it to my plugin?
Please help me, if you know!
Just look in the codex for shortcodes. There is the answer to your question.
If you have any more specific questions when you're trying to add them to your plugin then ask those. Not a general one like this.