With the D3100 auto iso keeps working when flash is used which means no unpleasant surprises so I would recommend :
Program mode - allows you to choose if you want to use flash or not and automatically adjusts for varying conditions .
Matrix metering is most consistent .
Auto iso : set at minimum 1/125th sec shutter speed.You can fix noise but you can't fix motion blur .
Flash : if you have an external flash use it in TTL-BL mode and use bounce flash if possible .
Keeping the lens at 18mm where possible will allow your lens to use its widest aperture of F3.5 to capture more light .
Another good place to learn , especially with a Nikon is nikoncafe