My Toilet Bowl Will Not Fill To A Proper Level. How Do I Adjust?

The most likely problem here is simply the floatation ball is the cistern is set incorrectly and needs altered.

After a flush, the tank begins to refill. It stops automatically when it reaches a preset level. In nearly all cases, a float is lifted by the rising water. When the float gets high enough it closes the water valve. If you lift the lid on the top of the toilet tank and flush the toilet you should be able to see quite clearly how it actually works. This floatation device is easy to see hard at work and from this you will quickly see whether or not the problem is the level at which the float is set.

In order to adjust the water level, you must set the height at which the float shuts off the water valve. There are several styles of floats. In older toilets, the float hangs at the end of an armature, along metal or plastic rod. Where that arm connects to the water valve there is usually a screw. Adjusting that screw will change the level at which the water valve shuts off. If there is no screw or it is already at its maximum, you may be able to bend the metal arm but you should remove it from the valve or risk damaging the fill valve.

In newer toilets the float is often a collar around the fill valve and rides up and down vertically. The float typically has a spring clip connecting it to a metal rod which connects to the valve shut off control. Squeeze the spring clip to slide the float up or down the metal rod.

Hopefully this should fix the problem, if not it may be a blockage somewhere in the system, for example in the diaphragm of the fill valve.

Coantman says it is a clogged bowl and I believe he is absolutely right.
You can try plunging it with a good quality plunger which you can purchase at any hardware supply.
If that doesn't work,you may have to run a cable, commonly referred to as a snake, down the drain to try and get it unclogged.You can rent one at a rental supply store/company.

I had the same problem and it was caused by the vent being clogged. It happened this past winter when an ice storm hit. Once the ice melted the toilet worked normally.

You may not have a float "ball" but you have a float. It's on the main vertical shaft where the overflow water runs. You can move it up and down. Down will allow water to fill the tank. Up will stop the flow. The higher up the float is, the fuller the tank will be. All you have to do is loosen a screw, reposition the float, and retighten the screw.