Ok this is pretty basic stuff but I am gonna try and give you some pointers rather than write the code for you. You have 2 ways of achieving this:
2) ASP.NET AJAX Control Toolkit: This is what I have used in the past and to some extent in the present as well. It has a control called ModalPopupExtender that displays a pop-up dialog, have a look Here
]. You can build a panel containing the login controls as a separate user control and add that to the modal popup extender. You will then have to raise an event when the login is successful to redirect the user to the corresponding page but if the login is unsuccessful, keep the pop-up open and prompt the user for correct credentials.
However, I would not recommend using pop-ups because on mobile phones and smart phones pop-ups tend to be quite annoying. They tend to be a bit annoying even on desktops and hinder user experience. I would rather recommend using in-page login mechanism so the user interaction is consistent throughout. Unless, you can figure out a way to optimise pop-ups on a mobile phone.