until the when you think the shutter speed is too slow and causing your image to blur, than increase the ISO.
Shutter piority is good if you want quick shots.
If not,go for Apeture model and get a tripod.
You shoot with S mode because you need to fix a shutter speed high enough to freeze the action required for the scene.
E.g. basic sports may need at least 1/125 sec and this may cause a shallow DOF. And at a lower light level, you may also need to increase the ISO.
I'd suggest you try out the A mode first.
Hi msbinary,
If you're not comfortable or not sure what to do, maybe you can consider strating with P (Program) mode. With Nikon camera you can still control the shutter/aperture while in P mode by turning the dial at the back (Nikon calls it the "main command dial"). You'll see a "P*" on the LCD indicating you're in "P-shift" mode. To get out of it, turn the dial until the "*" disappear. Once you're more comfortable with this, you can then shoot in either A mode or S mode.
Have fun!
Cheers.
Btw, you might want to take a look at this Photography Notes For Newbies written by Sulhan, one of CS' moderators. It was originally targetted for Konika-Minolta newbies but is applicable to newbies using other cameras as well.