While the f1.8 lens is a very good one, the first thing to realize about the lens you bought is that it's one of those which won't autofocus with the D3100 - I warned you about that when you first asked about the camera. Because the D3100 doesn't have a focus motor you have to be careful about what lenses you buy. The 50mm f1.8 AF requires a focus motor in the body. We've got two threads about that lens in the Camera Corner -
Got a new lens and
Fixed length lenses
You need to look for
AF-S lenses if you want autofocus with the D3100. There is an AF-S 50mm f1.8 -
the Amazon link is here - and that lens would work with the autofocus on your D3100.
Also realize that the 50mm lens is not a "normal" lens on the D3100, it's actually a short telephoto. What you might find more useful would have been
the 35mm f1.8 AF-S, which is both a normal lens and AF-S so it will autofocus with the D3100. If it's not too late to swap, I'd consider doing that.
The SB400 is a nice off-camera flash, and should work fine with the D3100. As with the built-in flash, you're going to want to be sure to set the D3100 to manual ISO200 when you use it, and use Aperture Priority (A) mode to pick an appropriate f-stop. I'd suggest starting with f8, and then when you're used to how the pictures turn out, experiment with wider apertures (lower number, less depth of field, more effect to ambient light) and narrower apertures (higher number, more depth of field, less effect of ambient light)
