Chain falls off all the time when I want to shift from small to big front ring
Your front derailleur limit screw is not adjusted properly. 

Limit screws literally limit how far the derailleur can travel in each direction, and are independent of the cable adjustment - I recommend checking the limit screws are correctly set before making any cable adjustments,and on the rear derailleur it is best to disconnect the cable before adjusting the limit screws. (do not disconnect the front derailleur cable as it is needed to move the chain guide) 

There are 2 limit screws on each derailleur, marked L and H. The H Limit Screw stops the chain guide from going to far away from the frame, so in your case it is the one that needs adjustment. 

To make the adjustment:


Put the  real derailleur on the outer most (smallest) cog. 
Using your hand, pull the front derailleur cable away from the frame to move the chain guide as far out as possible. Do not rely on the shifter alone as it may not be adjusted correctly, if necessary use a rag to protect your fingers.
Check the derailleur chain guide is approximately parallel to the chain and sits 1-2mm above the height of the large chain ring. If it is not then your derailleur angle and height need adjustment before adjusting the limit screws. 
Whilst keeping tension in the cable, adjust the H Limit screw so there is at least a 1mm gap between the chain and the guide. Tighten the screw if the gap is larger, and loosen it if the gap is smaller or the guide is touching the chain. 
Rotate the pedals a full 360° and check this minimum 1mm gap is maintained at every point of the rotation. Many chain rings are not exactly straight, a small deviation is nothing to worry about, but a large deviation means the chain ring or crank may need replacing, or the bottom bracket needs maintenance. 


Your H Limit Screw is now properly adjusted. You should now check the L Limit Screw and then set the cable tension so you shifter works correctly.