As the friends have written the basic noise factors are
1. ISO setting
2. Possible Underexposure
3. At some cases ambient temperature or sensor heat up, but in rare or special occasions such as long exposures
1. ISO setting
Ideally base ISO setting, depending on camera, is the best choice depending on lighting and scene conditions.
2. Possible Underexposure
Keep your mind on the histogram. It is one of the most useful tools in digital camera era. There is also the ETTR technique (Exposure To The Right, without blowing out the highlights of course)This technique will give you a file with high signal to noise ratio useful even when shooting at slightly higher iso settings having in mind of course that as you go higher to ISO sensitivity you will loose in dynamic range, especially in the shadows area. Keep in mind that the blue channel is more prone to noise than Green and Red.
3. At some cases ambient temperature or sensor heat up, but in rare or special occasions such as long exposures
It can affect the result in some cases but rarely. Mirrorles camer are likely to be more prone to these (in an old technology design) or even to DSLR when used for a long period of time in live view. But as I said this will rarely occur....
I'm posting a video for the ETTR technique
And an article.
Exposing to the right, or ETTR, is an approach to photography that is as helpful as it is controversial. On one hand, exposing to the right is yet another technique to remember while shooting, and it can potentially ruin your exposure if utilized incorrectly.
photographylife.com