You've got to separate the layer you want from the rest of the background. There are lots of techniques. The easiest way is to have your solo talent wear something that has a unique color and then use the Leave Color effect. The other way is to duplicate your layer and rotoscope your hero to separate him from the background. If there are elements that pass in front of your hero you have to do some roto work there also.
There's a great tutorial on rotoscoping (done in Commotion but the principal is exactly the same in AE) HERE. Be sure to check out both Part 1 and Part 2. You'll find links on the left side and if you click the title of the tutorials at the top of the text you'll get a video that explains the process. These are the best explanations of the process of separating elements in video footage that I've ever seen.
You might also try a Google Search for leave color tutorial in AE or something like that.
As Rick said, the simplest way would be to dress them properly in a color that is so unique it doesn't appear in the surroundings. What color it is, is mostly not important as long as it is unique - even under worst circumstances you can turn a red shirt into ablue one with a hue shift and some contrast adjustments. it jsut needs to stand out, including the person's skin. Then you can use effects like Leave Color to only keep the shirt and desaturate the rest, or extract the area with other color correction and keying tools, sometimes requiring you to work with duplicates of the same layer and inverting the resulting Alpah channels. If that's not possible, hard roto work is usuallythe only way to get this kind of thing, but obviously there are many levels inbetween where you may be able to extract parts of the color the easy way and only have to touch up the missing regions... Anyway, again, more than anything else it depends on how you shoot your footage.