I'm unable to get a clear picture indoors when I zoom in on something.
I'm not sure what you mean by this ... can you post a pic? The S5 should be clear and sharp at any zoom level indoors or out.
By "clear", however, do you mean "not noisy"? Is the picture sort of grainy?
If so, the only way to reduce the noise is to shoot a lower ISOs (which isn't always possible indoors without a flash). You need to switch to one of the "creative" modes (P, Av, Tv or M) to set a specific ISO (it's got it's own button on the back, but it doesn't work in Auto or Scene modes).
The higher the ISO, the more sensitive the camera is to light, but the more noise you get in your pictures.
I can't take a clear (non blurry) indoor picture without a flash.
That's probably because you're shooting in low-light conditions. The blur is either camera shake or subject motion, and it's visible because of slow shutter speeds. Without a flash, you might be getting shutter speeds of 1/4 of a second or less, and it's very hard to hold a camera perfectly still for that long (without a tripod). If you're shooting static subjects, using a tripod will help. If you're shooting moving subjects, a flash is the only real option.
When the light is low and you don't want to/can't use a flash, one of the best options is to switch to Av mode, set the ISO to 400* (or 800, or 1600 if you're desperate for the shot) and set the aperture to 2.7 (or as small a number as you can get for the zoom level) and the camera will pick the fastest shutter speed it can. Sometimes, switching to spot metering helps if you've got lit subjects against a dark background (like a stage show or concert).
Half-pressing the shutter button will display the shutter speed the camera is picking in the lower-left corner of the EVF/LCD ... if it's too slow (anything less than about 1/30 is really hard to get a sharp picture with) you can try increasing the ISO (if you're not already at 1600) or decreasing the aperture number (which is making it a larger opening) ... you might have to zoom out from the subject to decrease the aperture value (2.7 is as low as the setting goes, at the wide end, 3.5 at full zoom).
Remember the S5 is a great camera, but it's not a $1000 DSLR with a fast prime lens.
*using ISO 400 or higher, you'll probably want to use Noiseware or something similar.