I'm not an Olympus user but the questions you are asking apply to all cameras.

Even with your 50mm, f/1.8 you may not have enough light and may have to push that iso up.
Your settings are fine only if the amount of light available will allow for it.
Do your pics come out dark?

Yes, the "f" makes a difference in depth. "f" is your aperture or how far the shutter opens to capture the light. A small aperture needs more light and a large aperture needs less.
Thus you can use faster shutter speeds with a larger aperture.
The numbers can be confusing for a new shooter. Small apertures have larger numbers and large apertures have smaller numbers. On your 50 mm f/1.8 is your largest aperture and allows for faster shutter speeds. Probably f/22 or 32 is your smallest apertrure and needs more light. What is used depends on the amount of light.

As far as depth goes, the largest aperture, f/1,8, on your 50, has the least amount of depth of field. Things in front and behind your focus point will be less in focus.
As you decrease the aperture, more will be in focus, but then you need to increse the shutter speed.

I'd only be guessing whether the settings you are using are correct for what you are trying to get and what you are dissatisfied with.
Post a couple of pics. Preferably right from your computer so we can read the exif data.
Exif will say what the aperture and shutter speed and other pertinent data are.