One of the most common things that ruins a photo, specially on low light conditions, is the ISO setting. Most of users take their photos on Auto mode and the remaining usually use a pre-set scene according to the situation. Only a very small number of user really set a custom ISO. On low light conditions, the camera tends to choose a high level of ISO, producing a photo with a lot of grain or even artifacts like colored pixels. What is ISO?
ISO setting change the sensibility to light of your camera. You can control this since most of the phones allows you to set the level of ISO, even if some in numbers (like 100, 200, 400, ...) and some in description (like Low, Med, High). Higher the number, more sensitive is the camera to light but more grain is expected on the photo.So, why choose the ISO?
By selecting an higher ISO, you will get more sensibility to the light and you will be able to catch more light and the phone will automatically choose a smaller exposure time (even this varies with the phone model). If you are shooting in low lightning conditions and have moving objects, it's not a great idea to choose a low ISO as you will most likely end with a blurry photo (unless that is the effect that you pretend). On the other hand, if you have still objects and you want to control the level of noise, then you should set a low ISO. You can try compensating lowering the ISO by using an higher level of EV (usually available on mobile phones). Even if you end up with a under-exposed photo, you can edit later and play with the brightness, contrast and gamma settings to overcome the dark photo you got. One thing is certain, once you get noise on your photo, the detail is gone.
In the end, you will learn that Auto ISO might be making your photos with grainy aspect and without that need, because you could have chosen a lower ISO and still get the amount of light you wanted to your photo. The trick here is making some tests and training to see how to get the best results for your phone model.Tip
If you want to post process your photo to reduce the grain, there are softwares for that. I would recommend you this one: http://www.neatimage.com/
that has been demonstrated in this topic: Removing noise from photos (Post-Processing)