You find this text in www.rpgmakerweb.com/support/products/faqs
Add a "credits" page at the end of your game and credit all the assets you have used. Check what license assets have (usually there is a "read me" with them if you bought them or downloaded them for free). Assets bought on RPG Maker business site can only be used with RPG Maker, but you can also use external assets in RPG Maker. Don't use assets if you don't know the licence.
Hope this helped. Happy game making!