To determine where the water is entering, lock the door then use a hose pipe for a few seconds on the outside. If the bottom of the frame is full of water when you open the door there is a drainage issue.
Open the door and pour water into the drainage channel at the bottom of the frame. Check that the water drains away quickly through the external drain holes. If it doesn't then remove the external drain covers (if fitted) and remove dirt etc. using the end of a wire coat hanger. Clean out the drainage channel and the drainage slots and flush through with water. Sometimes the drainage holes are too small and need enlarging using a drill. The drainage holes need to be at least 5mm wide by 20mm long.
Check the door pulls tight to the seals when locked. If not then adjust the locking points and keeps accordingly. Damaged seals may contribute to a leaking door but the cause is normally due to poor drainage.
If this doesn't help then please supply more info with regards to exactly where the water is coming in.