I lost my iPhone, how i can switch on “find my iphone”? Thanks!

This support article tells you how to do that and more:

Click on the link and read it to find out what to do "If your iPhone, iPad, or iPod touch is lost or stolen" --> https://support.apple.com/HT201472

If Find My Phone wasn't turned on before you lost the phone, you can't turn it on remotely.