You can start making basic "redstone" devices as soon as you have enough wood to make a door and a pressure plate: make the door and the pressure plate, and put them next to each other. Voila! Your first "redstone" device without needing any redstone.
If you want to start getting into more complex devices like flip flops, you need redstone dust: you can find it by mining redstone ore deep underground: 16 layers above bedrock or lower (sea level is at the 64th layer).
The best way to learn how to make redstone devices like flip-flops is to watch the myriad of tutorials on YouTube: figure out what you want to make, search for a tutorial on how to make it, and repeat it1.
For example, Rolf-David's tutorial shows you how to build a basic T Flip Flop:
The basic flip flops (and most redstone devices) are pretty simple, but aren't very efficient because of the way redstone works: they take up a lot of space and because power doesn't travel instantly across redstone, larger circuits are incredibly slow.
So what people have been doing is exploiting buggy implementations of blocks not intended for redstone circuitry to make them smaller and faster. Etho is probably best known for exploiting what's called the "block update"; a quirk of Minecraft where blocks send power when they update their status:
For the most part, if you never got into advanced features like this, you'd still be fine.
Note 1If you want a more comprehensive education on Redstone, there are couple of channels that focus on it pretty heavily: Etho's Lab and SethBling probably the two biggest ones. Etho's Lab is more "Let's Play"-ish (although he does do stand-alone tutorials from time to time), whereas SethBling is pretty heavily focused on tutorials.
One easy(ish) solution is to use a sand door. You can make them as wide as you would like, and about 3 high.
Here's one design that should work with the XBOX edition:
If you want to do some searching, what you want is a tileable triple piston extender for Minecraft 1.0.1. Or, if you only need it to be 2 high, then you only need a double piston extender, which is quite an easy design.
Best Answer
The most simple door that a villager cannot go through is an iron door powered by a button. If you need to use levers, you will need to build a simple XOR Gate.
Buttons
You will need:
Put down the iron door and place a button on an adjacent block to the door on both sides. When you press the button the door will open momentarily, with just enough time to walk through.
Levers (XOR Gate)
An XOR Gate is relatively simple to make, and allows you to open the door using levers on both sides of the door. I am using XOR Gate Design D from the wiki.
You will need:
In our case, the output the XOR Gate is toggling is the iron door.
I chose this design because it is compact and simple (doesn't involve repeaters or pistons), but there are other XOR Gate options to choose from if this one doesn't suit your needs.
With both of these methods you can open and close the door by flipping the lever or pressing the button. Villagers cannot get through becuase they cant interact with buttons or levers.
Quote from the "Door" page on the wiki: