First off, you'll need to learn some java. I know this isn't what most people want to hear, but unless you just want to create various "new block" plugins, you'll need to understand how the code actually works and NOT just which words to switch out to change the colour or strength of a block and whatnot. Find a beginners guide to java. You could look for online websites, but I would recommend books instead, as they're better. You can download e-copies of many for free. Then, once you can make a few basic programs on your own (should only take a week or so to get to this point, if you read a bit each day) and have a basic understanding of what symbols and structures java uses, go into the minecraft code and start reading it, "decoding" it and figuring out what each part does. (There are many guides on how to download and use the Minecraft Coder Pack to decompile Minecraft into readable files.) Experiment and make couple simple mods, just to get the hang of it. THEN, you should head on over to the bukkit forums, and from the resources there you can learn how to adapt your mods to multiplayer plugins.
NOTE: I haven't made any minecraft mods since before the internal servers and new modding support (I think?) were added. So my experience is circa 2011/ 2012.
