To keep your swimming goggles from falling off, you must wearing swimming goggles with the proper size to fit for your face and head. The strap pf the goggle shall go over the crown of your head. Make sure that the nose piece is adjusted to the right size. If the swimming goggle is not the size of your face and head, you need to buy another properly pair instead.
I think the strap of your goggles is too loose so that it can't keep the goggles on your face, which cause your goggles to keep falling off your eyes. In this case, you can adjust the strap by yourself to make the goggles fit onto your face properly. Or the goggles you wear are not in right size so that they cannot fit you properly. If so, you should take your goggles back to the place you bought them to have them re-checked.
It's very easy. 1. Getting your arms close to the ears and lower your head .2.entering the water slightly curl .you must remember that arms close to the ears! 3. your head, cervical vertebrae and back keep a straight line.4.practice . I suggest that you first don't jump but hunkered down in a start slowly, even leg and focus moved forward and feeling you will enter the water now getting your arms to ears, and then getting into the water.practicing more, and you will find your goggles won't fall down and you can even diving!