someone has stolen my device but i know icloud id.How to switch on find my i phone

You can’t turn it on remotely, you can only do it on the phone. If you didn’t there is no way to track it. However, it is on by default when you set up the phone, so it may be on unless you intentionally turned it off. To check log in to https://icloud.com/find (be sure to include the “/find”, or click on the link in this post).