I've had a few people contact me asking how to make a mod. It's not really an easy thing one person can explain and if I went into every step, I'd probably be here all week. Others have posted info on how to do this stuff anyway. However, it's sometimes hard trying to find that info, so I'll give you a break and try to include everything here for quick reference so you can get started and won't have to waste a bunch of time trying to search for the right things.
First, you will need the Java SDK found here. You'll have to configure it to your computer, which the following video explains how to do that. Ignore the fact he's using a different version of JAVA and after you install it, you can skip to about 5:50 in the video to learn how to configure it. That's the main tricky thing about this. Here is text tutorial on how to configure Java if you're still having problems.
You'll need the latest version of Minecraft MCP, found here and you'll need something to edit code on, which I suggest using Eclipse (I'm using the one that says "Eclipse IDE for Java EE Developers". Here's a video that shows you how to configure those to work...
There's a lot of good tutorials under the Mapping and Modding Tutorials forum, but this thread helped me get started the most. If you plan on making new mobs, you'll save a lot of time and frustration if you use Techne. Also, the IRC channels #mcp, and #mcp-modding are good places to ask other modders for help. There's also some good tutorials if you search on YouTube.
The hardest part IMO is just getting Java, Eclipse, and MCP set up so you can start coding and then it's pretty fun and easy from there on out (depending on how well you can figure out Java code). I started out with no knowledge of Java a few months ago and found all these tutorials others had done and with that I was able to make NamCraft. While it looks like a lot to learn, well, actually it is, but just keep at it and don't give up. Just remember, if some hillbilly, high school drop-out, from Arkansas can do it, anyone can do it.