How do I get this Mac G5 up and running ? (boot to OS)
Software - You need to get an old OS X install CD, boot from it, and install. Depends a little on the specific Mac you have, but you can check sites like eBay. You can find more info on sites like: https://lowendmac.com/
To open the CD tray - hold the left mouse button on boot - it should pop open for you.
Shoehorning RAM into a slot doesn't sound healthy. If it came with a stick of RAM, take out your new RAM and leave what was there in it, and try booting. Putting in an unknown stick, at least till the box is up and running is going to be another potential headache.