You will want to find a good quality rubber glue/adhesive that can be found at any good hardware store.
A cut in your wetsuit is most commonly made by finger nails or a sharp object when the neoprene is stretched.
Hold the cut open as much as possible so that the glue can get deep inside the cut.
Using a toothpick or q-tip put some glue inside the cut. Be sure to put glue all over both edges with not too much excess glue.
Let glue sit in the cut with the cut open for about 4 minutes. This allows the air to activate the glue.
After the glue has had time to bond while sitting open, firmly press down with your finger to ensure the two pieces match up.
The end product will look similar to the picture below. The suit will be left with only a small scar.