First, have you run Software Update and installed everything that comes up? If not, start by doing that.
As far as Flash, the installation process is very simple: Type "flash" into the Google search box and hit return. Click on the top link, which should take you to the Flash installation page. That will automatically detect the correct version for you. Click download, run the installer, and follow directions.
(Note, by the way, that if you're running Snow Leopard on your computer Flash player comes along with it; this might be true of regular Leopard 10.5, too, but I'm not sure.)
If you're still having the issues the easiest thing to do is download and install ClickToFlash:
http://rentzsch.github.com/clicktoflash/
...which will stop Flash junk from automatically loading until you click on them. This prevents the Flash player from using 100% of your CPU and causing the fans to ramp up every time there's a Flash ad banner on a page, which is pretty much always.
If it is Flash that's causing your Safari crashes, that'll fix it.
