still in the full learning process, but yes. I can never get a good ISO. It ends up to grainy, and then when i use the flash it shows up to much. If i then set the apeture for more light to be needed them its blurry
1 -What ISO are you using? The D5100 should be pretty much noise free up to at least 800.
2 - A bigger flash well help you with that not due to the larger size, but due to the fact that the light could be bounced, or better yet, taken off-camera.
3 - Blurry or fuzzy? A larger aperture (lower f-number) lets in MORE light, which reduces blur. If it's fuzzy, on the other hand, it's probably not in focus. Large apertures have less of the picture in focus, and cameras have a harder time focusing in low light, so that may be your issue.