external flash and how the camera reacts to it

Have had the d5100 for about 6 weeks and enjoying it tremendously as it provides stunning pictures. I've moved into mostly full manual "M" mode practice and liking the results thus far. I'm slowly getting more confident in understanding the light meter and how it relates/reacts to shutter and aperture changes.

So now I have this YN560II external flash. Is the cameras light meter now useless? The meter never leaves the max underexposed area. I assume with flash, I'm pretty much on my own? Not ready to invest in a real light meter just yet.

I am taking practice photos changing the flash strength/direction/distance, apertures, and shutter speeds with good results. Supposed to be taking some important shots next week during the neighbor kids graduation and party after. There will be indoor, outdoor, and night shots. For lenses the 18-55mm kit lens, 35mm 1.8G, 55-300mm G. Any insight or tips always appreciated.

It's obviously an important family event and I would like to wow them.....

Check out Youtube you should pick up some good tips....I take pics to WOW me,

Shoot raw if you don't do that already, the extra captured dynamic range will be great for recovery for your under or over exposed shots.

When using any type of flash, whether speedlights or strobes. You will always have two exposures to now deal with. You have ambient light exposure. And you have flash exposure. By adjusting your shutter speed, your adjusting your ambient light exposure. By adjusting your aperture, your adjusting your flash exposure. So say your taking
portrait of someone outside on a sunny day. Your subject is in the shade, but your background is in full afternoon sun. If you take the shot exposing for the background than your subject will be underexposed. If you expose for the subject than your background will be overexposed. Thats where flash comes in. Most people would expose for the background and fire away. If you need to adjust the flash on your subject than adjust your aperture. If you need to adjust the background than adjust your shutterspeed.
Of course theres a little more to it than that but thats pretty much how it works. Your best bet is to find some you tube videos on Brian Peterson on understanding flash photography.

Thanks guys. I have been youtubing it and am learning lots of new stuff. Using flash on sunny days is called fill-flash? See, I'm am absorbing some. At my age though, for every new thing I learn, I have to forget others to make room.

It has been about 7 years since I've upgraded cameras and wanted to make sure I know this ones limitations and capabilities. This is my first DSLR.