If your laptop has an external VGA port, often found on the hinge side, you
can connect to an external monitor via a standard VGA cable. Often, the
notebook will then drive both the internal and external displays.
Your best source of information will be the notebook maker. I suggest you go
to the maker's web site and check their support offerings. If you've lost
your manual, you can often find a download.
Post the maker and model here for additional assistance.