Somewhere on the camera there should be a "depth of field preview" button.
When you look through the viewfinder, the lens will be "wide open" - the biggest aperture / smallest
f number the lens is capable of. There's a button somewhere that will "stop" the lens down to the currently selected f-stop, letting you see the depth of field (what's in focus) that the picture will be taken at. The tradeoff is that as the aperture gets smaller, less light comes through, so what you see through the viewfinder gets darker. But, it can be fun to play with.
There also might be focus indicator numbers on the lens body itself. There's a hash mark in the center, and going out on both sides of that various
f numbers. These will tell you what range of distances will be in focus at what f-stop. I'm not sure if the fancy new digital lenses still have those markings on them. If they do, they're probably really small and hard to read, unlike the old school film stuff.
f-stop is a fraction of some sort, hence it being written as f/2.0 I believe it's the ratio of the aperture diameter to the focal length. But I could be off on that.
Whoops, forgot to mention one of the more concrete examples of f-stop / aperture stuff. Big
f number, small aperture (hole). Taken to an extreme, you have a pinhole camera (or point and shoot) effect... everything is in focus all of the time (pretty much). The other extreme, really big aperture, only a tiny amount of the picture is in focus.
f/4 is pretty wide open for a zoom. f/2 is reasonable for a cheap prime lens. Below f/2 gets expensive (usually).
Zoom lenses will usually have a range of biggest aperture available depending upon the focal length you are at (how zoomed in).
