I'm also too late for your project, but here's a couple of things to note:
You were getting the same lightmeter reading from your friend's white face and from the 18% grey card because 18% grey will approximate the light reflected off a white person's skin. So, it simply means that your light meter is accurate and that your friend has average caucasian skin tone. If you are filming someone of any other complexion (i.e. darker skinned), you may need to open up the aperture a little more.
Your internal light meter will be averaging the exposure for either the whole frame, or it will be weighted towards metering the centre of the frame. You can tell by aiming at a really bright object against a darker neutral background. First, centre the camera on the bright object. Then, wiggle the camera around. If the internal meter immediately drops down when the bright object is not perfectly centred, then you have a centre-weighted meter. If the meter does not drop down until the bright object starts to go out of frame, then you have a frame-averaging meter. Once you have decided which kind of meter you have, you can judge more accurately what the camera is metering for.
No matter what kind of meter you have, the best thing to do is to zoom in on your subject to get a more accurate reading. Of course, if your goal is to simply expose the faces correctly, then zoom in on the faces, set and lock the aperture, and zoom out to your desired frame. To get a better idea of how your final image will be exposed, zoom in on different parts of the frame. Zoom in to the darkest shadow, and note what the aperture reads as. Zoom into the brightest highlight, and note what the aperture is. Zoom in to the subject that you would like to expose for (or anything that is the same colour and in the same lighting) and note what the aperture is. With these three numbers, you will be able to answer some questions: Are my darkest or lightest areas radically different than my subject? How much of the final frame is like the darkest, lightest or middle exposure? If you are shooting reversal film stock, there is very little leeway for exposure. If your brightest or darkest areas are more than 2 or 3 f-stops different than your subject, they will begin to disappear into blacks and whites. If you are shooting negative stock, you have a lot more leeway, and will be able to see deeper into the shadows and highlights. The formal way to control exposure in this manner is called "the zone system". You needn't be as rigorous as the "zone system" but it is a great way to think about exposures and consider your choices as more than a single f-stop setting for a whole shot. Looking at your exposure this way is especially important to achieve the look you want when you are shooting wide shots. (By wide shot, I mean anything where you can see a full human figure or more.)
If you can get a cheap external meter, they are very useful. You will most likely get an incident meter, which means that the meter is designed to be held where your subject will be. The light that falls on your subject should fall on your meter, with it facing towards the camera. To get a better sense of the lights and darks in your exposure while using an incident meter, you will need to walk around the shot, metering in the darkest areas and the brightest (basically, walking around to the same places I had you zoom in to when you used the internal camera meter above.) Everywhere you take a reading with the incident meter, it should give you the same reading as though you had held an 18% grey card in that spot and zoomed in to check exposure with your camera.
There are lots of resources out there, plenty to get you up to speed quickly. The most useful ones for you at this point will actually be books on still photography. They will get you up to speed on metering and exposures. I also recommend Lenny Lipton's book, if it's not already on your syllabus.
Best of luck,
Dave
