I have a video and I want to create focus on a area close to the camera but the background to be blurry. I have watched a video from Adobe about it but it only explains what the effects does and not how to use it. I mentioned something about getting the data from a blur map, which I don't know what is... Can someone give me a basic step by step instructions on how to create camera lens blur. I've only just started After Effects so try to make it as basic as possible ;)
The "blur map" is a greyscale image, where the brightness of the image varies with the distance of the thing in the shot from the camera. Usually these are generated from a 3d app, like Cinema 4D, to allow you to add camera blur to computer-generated footage. Using one image to influence the intensity of another filter is sometimes called a "compound effect."
Since you are working with real-world footage rather than CG renders, you will need to do some rotoscoping (as already suggested). It is possible to generate a depth mask (a custom version of the greyscale image that a 3D app can generate) but in most cases it isn't worth the trouble -- separating elements with duplicated footage and animated masks, then blurring each layer the desired amount can often do the trick. And it might be less work (though it will still be a lot of work!); good luck!