You need to use a partition manager such as the GUI application GParted. You need to run it from a LiveDVD because it can't work on mounted partitions. GParted is on the SL 5.0 GNOME LiveDVD or you can download the ISO for SystemRescueCd
which also has it. The command line tool sys-apps/parted is also on the SL 5.0 GNOME and KDE LiveDVDs, but is not newcomer-friendly. Unfortunately there is no GUI partition manager on the SL 5.0 KDE LiveDVD.
Basically you boot the LiveDVD, launch GParted, and the GUI will appear and GParted will find all your HDDs and partitions. You can drag partition bars to resize them, and so on. The drop-down menus are self-explanatory. GParted is easy to use but, if you need any tips, see the documentation and screenshots on the GParted Home Page: http://gparted.sourceforge.net/