they make them with USB also, if im not mistaken.

I only see one plugin and i guess it's for headphones . how to use an external mic in this case ?

Hello. For an explanation of the single audio jack socket, see the Combo/Headset Jack section of the Headphone Jack FAQ.

Thanks guys for your replies , I have now a headphone with one plugin I mean the mic and speaker are in one plugin because you told me that dell supports both in one plugin now , but still doesn't work , what should I do now

Hello. Try re-installing your Realtek audio driver. If the mic does not work and you think you have the correct type of headset, contact tech support for service.