If you flatten the image you can save as jpeg. Jpeg also has to be 8 bit RGB.
What seems to do the job for me is File - Save As, and choose the format I want (e.g., JPEG).
Another option is File - Save for Web.
These answers are clearly obvious, so maybe it would be good if you would describe why you aren't able to do so (and what version of Photoshop you're running on what OS). Older versions of Photoshop had more stringent requirements of the document (such as what Curt describes above) before you could save in certain formats.