Go to www.purezc.com and download a new tileset to start in.

If you want to rip in new graphics, Quest->Graphics->Tiles, then press the grab button.
I could spend half an hour explaining how best and how best not to use the grab tool, but seeing as I'm lazy I don't really want to.

Have a go working it out yourself and if you get stuck I'm sure me or someone else'll give you a hand.