This is the scary one, but well worth playing with as it will help you understand what changing various settings do.
In manual mode you have to set the aperture setting yourself and you have to set the shutter speed setting yourself. If the resulting exposure is wrong, then you are to blame, if it’s right then that’s a feather in your cap and you deserve a pat on the back.
So how do you know what are the right settings? That’s when experience comes in, and it is not something that can be learnt quickly. Experienced photographers will know that on a sunny day, with the camera set to an aperture of f8, that a certain shutter speed will generally give them a decent result but as there are so many variables to think about, it is a tricky thing to do. Luckily with digital you can experiment as much as you want without worrying about wasting film and money on learning and making mistakes.
As manual mode involves setting two parameters, it takes that bit longer to set the shot up, so if a quick shot is what’s required then a mode like aperture priority or shutter priority might be the better option so you can concentrate more on getting the picture rather than worrying about settings.
On the other hand, if you have the time to set the shot up then using manual mode will teach you the most. Set the aperture and shutter speed, take a picture, review it and see if it looks ok. If it’s under exposed (too dark) then you might need to slow the shutter speed down a bit or maybe use a wider (smaller) aperture setting. If it’s over exposed (too light) then you will need to do the opposite, speed up the shutter speed or reduce the size of the aperture by choosing a larger aperture value.
I don’t want to dwell on manual mode too much at the moment, it’s a bit like running before you can walk, but I thought it needed to be mentioned. Don’t be scared of it, just remember that it needs a lot more thought and experience to get right.