The RTP stands for 'run-time package' and is just basically another term for all the assets preloaded into a new project you create.
The door sounds are OK, I was referring to the default UI sounds (when the player is menu-ing etc). These can easily be changed under the SYSTEM tab. Changing it to pretty much anything suitable would be an improvement over what the project defaults to is a massive plus in my eyes.
Fonts must be 'True Type' fonts. You install them as normal then move the font file in to the Font folder in your RPG Maker game project folder.
Then go into the SYSTEM 2 tab in your project and type the font file name (make sure it's suffixed with .ttf) and it should load the font into your game.
Windowskins you just find free stuff with a quick Google search, then copy/paste it into System folder making sure the filename is 'Window' that will replace the default one.
These are quick guidelines but there are in depth tutorials knocking about if you struggle.