Wrapping Your Head Around Lighting
Lighting can be a big hurdle for beginning filmmakers (or even experienced ones!). How do you make a light look natural? Isn't the room lighting or sunlight good enough?
I've found that in most of the classes and workshops I've taught, lighting tends to be the first things to go, when the shoot runs into road blocks. For most students lighting just feels like a nice extra step, but it often gets forgotten about so long as the room light is bright enough, and you can see what's going on.
However, intentional lighting choices really can take your film from the "my first film look" to "hey, that looks pretty good".
Here are a few tips I've learned over the years of how to light your film (whatever the genre) in a professional looking way, no matter what gear you have available.

1. Consider the Source - start there and work backwards
Where should the main light be coming from in this scene? The sun? An overhead room light? A lamp in the corner? This light is known as the source light. Knowing where the majority light is coming from in your scene helps you make decisions that look natural and make sense for the environment you're in.
2. Create a bit of contrast
Go look up some shots from your favorite movies or shows and you'll start to notice a pattern in the lighting choices. In the majority of the lighting, there is one side of a person's face that is typically brighter than the other. This creates a nice cinematic contrast in the person's face and keeps them from looking "flat", or one dimensional, onscreen.
3. Have the brightest light on the subject's face be opposite from the camera
If you take a look at lighting from a film, you will probably start to notice that the brightest light on a person's face is typically the far side of the face from the camera. This comes from a technique used in filmmaking called 3-point lighting.
This means it usually looks better to position your subjects so that the brightest light in the room, or the source light is on the opposite side of camera to them. If you're filming outside, it's easiest to achieve this by moving your camera and subject place according to where the sun is and use the natural sunlight to your advantage.

In this screenshot the source is a ceiling light. I positioned the actor so that the light was on the far side of her face (see diagram blow).

In the documentary scene below, I used the bright sunlight as my main source light. I positioned the painter so that the sun was lighting up the far side of his face.

4. Use your source light as a jumping off point
The goal of lighting is to create a believable environment for your story. The easiest way to do that is to "motivate" your lighting choices so that the light is coming from an identifiable source.
For example, take a look at the photo below. It looks like most of the light in the room is coming from the bedside lamp. However, in reality the lamp itself wasn't bright enough to light up the actress for the camera, so I needed to add some more light. I used a separate light, just offscreen that lit the actress from a similar direction as the lamp. This creates a natural look, because we already expect to see light coming for that direction.

5. When in doubt, try soft light
Try working with the source light you have available. Can you use that light and bounce it off another surface? If you use a bounce board (reflector disk, white piece of Styrofoam, table cloth, really anything that's white) the light will reflect from that surface, and back on tho the subject. By bouncing off the surface, this will scatter the light and create a softer glow on the subject's face.
This is called creating a soft light. Soft light is any light that doesn't cast a harsh shadow. This is usually because it's scattered by passing through an object (for example a piece of fabric, or sunlight passing through clouds), or bouncing off a surface (like the styrofoam, or reflector disk in the above example).
A favorite trick of mine is to try bouncing the light off of a white wall or ceiling. Instant soft, bounced light that gently lights up the room.
6. Add a little something that separates your subject from the background
Sometimes a subject can look as if they're blending into the background if you don't add an extra light to outline their back, shoulder, or head. It really doesn't have to be a lot, but even a small light illuminating a person's shoulders or back can make a big difference to keep them from looking like they're one with the back wall.

I hope these tips help as a starting point for lighting your scenes. Like any filmmaking, the best thing you can do is go out there and start practicing. The more you experiment and try moving lights around, the easier it gets!