How to turn off the water flow to the toilet

That's a fairly popular fill valve. To open it:

1. Shutoff the water, you don't want a geyser.
2. Lift the float to the top position.
3. While holding up the float, push down on the top cap and twist the cap a quarter turn (I believe counter clockwise).

The cap will then lift off and you can inspect and clean any parts. Before reinstalling the cap, move any small parts to a safe location, get a small cup to place over the top of the valve, and turn on the water supply. This will jet water straight up from the top of the valve and clear out any debris.

Reinstalling the cap is the reverse of the above procedure. Put the parts back, position the cap back on top, lift the float, press down on the cap, and twist.

Note that if cleaning it doesn't work, it's fairly simple to replace this fill valve.

The water in the toilet seeks the level of the highest level of the high level of the trap, so if you raise the toilet bowl you must raise the trap the same distance. The trap is the curly, s-shaped part of the drain between the exit tube of the bowl and the wall. The highest level of the trap follows the line of bottom of the inner circumference of the trap where the the direction of the flushing water is parallel to the trap, as it is coming out of the climb from the bottom of the trap.

It could also be a vacuum in the drain sucking the fluid back into the drain if there is not a vent pipe running vertically from the drain to the roof to equalize the presscure.

I am not a plumber nor an engineer but a do it yourself person with a little math understanding from American public schools and a very basic, traditional grasp of physical science from experience and from the instruction of a peculiarly motivated chemistry professor at Bakersfield College, Ca USA.