Canon Rebel XS / Eos 1000D Digital Camera with EF-S 18-55mm f& EF 75-300mm f Lens "lloyds site" Questions

MY CAMERA (CANON 1000D) TAKES BEAUTIFUL DAYTIME PHOTO'S BUT CAN'T SEEM TO FOCUS ON A SUBJECT IN LOW LIGHT & AS A RESULT WONT TAKE THE SHOT. WHAT DO I HAVE TO DO TO TAKE LOW LIGHT / NIGHT TIME ...