DOES NOISE/GRAIN MAKE YOUR PHOTOS UNUSABLE?
One of the biggest things that photographers try to avoid is noise. (Grainy photos caused by bumping up the cameras ISO to help expose the photo). Is noise really a bad thing? No it's not. Noise is not something to be scared of at all. I hear people say things like "the D5500 can't handle much higher than 1600 ISO before the photos are unusable" this is absolutely not true. Do the photos have noise? Yes, but the photos are absolutely usable.
Obviously when shooting at higher ISO's the image quality, and dynamic range begins to degrade, but for the most part you really don't have to worry. If you shoot RAW, you can always edit and bring back some details in post. When you're looking at a photo and you're seeing noise, is it because you're zoomed in and pixel peeping? If so, of course your seeing noise when your zoomed in 1:1. That same photo looks great and noiseless when printed because you can't pixel peep a print.
One huge tip to keeping your image quality up, and noise down is to make sure you get your photo properly exposed in camera. If you are using high ISO and still accidentally underexpose your photo, bumping up the exposure in post will really quickly degrade image quality and create tons of noise. I've taken photos at ISO 12,800 that I've used because I exposed it properly in camera. If I shot that same photo at 12,800 and had to bump up the exposure in post, it would have been trash.
In summation, don't be afraid of using high ISO, and don't be afraid of a little noise. Just make double sure to expose the photo properly in your camera when using high ISO or you'll hate yourself when it comes time to edit.