Can I upgrade from windows 7 to windows 8 without losing any data if no then what to do

NOT recommended at this point as Win8 is in tech preview, i.e. Alpha release.
Still has a lot of bugs and other issues..

The preferred route is fresh install, so fire up a spare machine..

If you are talking about when Win 8 comes out, then you should be able to upgrade without losing data.

If you have obtained a copy of Win 8 now, it may not be a legal copy so it could be harmful to your machine. You do NOT want to upgrade an existing installation with beta (or less) software. Use a virtual PC or a test machine to do a clean installation.

Windows 8 ? That's still in development as per http://windows.microsoft.com/en-CA/windows-8/preview It should work like every other version they launched. You should be able to upgrade. Unless there is a problem with your hard drive in which case windows usually says to reformat. It is always recommended you back up your data. You can go out and get an external hard drive cheap and back it up there.