I'm using Canon 700d with 18-55mm and 55-250mm Kit lenses. Somedays my camera manual settings suits the environment and I'm able get some great shots.(Sea shore,Road,Terrace). But sometime I'm getting blurred, dark very poor shots, I tried with shooting many different setting but nothing helps.low-light(Indoor, using flash, evening). While I try to focus a subject it produces a brighter preview image but the resultant image is darker that it shown in preview. How to calculate the suitable aperture, shutter speed, ISO and white balance for the lighting condition and the subject. I do use tripod wherever necessary. Please advise.
First and foremost, you need to understand the "exposure triangle". In any given situation you can use these variables (aperture, shutter speed and ISO) to modify the final image. As you only have kit lenses (slow in terms of max aperture), you need to compromise often, set your aperture as wide as possible and you're limited to adjusting shutter speed and ISO.
Shooting in low light requires one (or more) of these things:
You can also of course add more light (strobes, flash, wait till morning), but adding more light is very situational and the result won't always be what you want as you're modifying the light and the mood of the shot. If not balanced correctly it will produce very unnatural look (think deer in spotlights kinda of look).
Your combo simply isn't suited well for low light photography. If you want to shoot often in bad lighting conditions, you need a fullframe sensor + fast lens (1.8 or faster). Fullframe can perform in higher ISO much better and fast lens will let much more light in for your sensor to capture.
There's really no way around it, you can't fool physics. Photography is all about painting with light.