What programming language do Minecraft plugins use?

I think you need to know java coding

Java. Lots and lots of Java.

http://thenewboston.org/list.php?cat=31
Do all those tutorials. If you have enough patience it's a really good idea to do all these ones after those: http://thenewboston.org/list.php?cat=25
If you don't do the first list of tutorials you'll have no clue what you're doing, so you must do at least those. The second ones will help you create actual decent mods, rather than just adding a new set of tools or something.

Once you've done all those, you'll be able to actually start creating mods. All those other tutorials were just for you learning the programming language to create mods. Take a look at the list of tutorials for modding here: http://www.minecraftforge.net/wiki/Tutorials

Java