At many points while making your mod, you may wish to refer to vanilla Minecraft's textures. Many mods will want to copy these textures and recolor them to make new weapons, mob variants, etc.
Luckily, instead of downloading them from the internet, you can very easily grab all textures from Minecraft with relative ease, helping you immensely with mod development.
Obtaining the Textures
First you need to download a program such as WinRAR, which will allow you to extract .zip files.
Then navigate to Minecraft's AppData directory. This is located at C: (or whatever drive you installed Minecraft on). C:/Users/(your current user)/AppData/roaming/.minecraft. If you cannot find the AppData folder, try selecting View at the top of Windows Explorer and enabling 'show hidden files and folders'.
Inside the .minecraft folder, open the versions folder and select the Minecraft version you are using, or simply the newest version. Then right-click the .jar for that version, and copy it somewhere like your desktop. Right-click it again and use WinRAR to unzip the .jar file.
After the unzip finishes, you will have Minecraft's inner workings in a folder. There's a lot of obfuscated code in here which you cannot read, but that's not what you're after. If you open the assets folder, and then the minecraft folder, and then select textures, you will find yourself presented with several folders containing Minecraft's textures, which you can copy and edit to your heart's content.
Blocks contains all block textures.
Entity contains all mob and entity textures.
Items contains all item textures.
Models contains armor textures.
Should you need them, the map and painting folders contain map icons and paintings respectively. The other folders are likely not of use to you.
And you're all set to make that Golden Zombie you were excited about!
A legal note
Make sure you can actually use Minecraft textures as using them might be against the Minecraft EULA in some cases. Retexturing textures or using them for reference does not break the EULA but redistributing Minecraft textures (allowing people to directly download them from you) or using them directly in your mod is not allowed.