I was going to say that I'm mixed on what to suggest, but I actually don't think I am when I really think about it.
I would recommend you start out with just plain MCP for Minecraft 1.6.4. Minecraft is pretty complex, and Forge adds a lot of extra complexity, both to the game as well as the development environment nowadays. I really think people should hop into pure, unmodified Minecraft, which they already know how the game is supposed to work. Change things, break things, and just have fun poking around at its guts. You can literally do anything you want to the game this way without restriction. Use Eclipse for your IDE (which MCP includes a workspace for) and you'll also have the advantage of the real-time compiling and code hot-swapping functionality, which makes development much more enjoyable.
The only reason I'm not completely recommending MCP for 1.7 instead is because it's not quite deobfuscated yet as fully as 1.6 is, last I used it. I ran into a few of these bits when porting mods and it threw me off at times. I don't think there will ever be a time when absolutely everything will be deobfuscated, and sometimes you'll simply have to figure out what a field or method is. But the more done for you, the better, especially for beginners. Though I won't outright discourage 1.7; if that version appeals to you more, go for it instead. You can probably manage to learn the basics of how the blocks and items and stuff works from either one.
As for Forge, the most important thing to know is that the development environment changed significantly for 1.7. Instead of using the same system for compiling/reobfuscating/etc that MCP uses, it now uses this new Gradle system, which can have its advantages and disadvantages. So just know that if you try using Forge for 1.6, you'll basically be learning how to set it up all over again when you go to 1.7. The other big disadvantage to using Forge for 1.7 is that you no longer have the ability to edit Minecraft's source to help test or debug things. I see why they did it that way, but I'm not a fan, and think it hampers more than it helps.
Anyway, this might be completely opposite to what other folks might suggest, but this is the kind of thing I would suggest to myself back when I had a hard time finding my way into it all.
