Pour juice, sugar and agar agar into a medium-sized saucepan and stir.
Turn on heat to medium and stir frequently with a whisk.
When the mixture begins to boil, turn the heat down and let it boil slowly for 5 minutes stirring occasionally.
Pour the jello mixture into individual serving glasses.
Refrigerate for at least 1-2 hours.
Notes
To make jello in a mold
Double the amount of agar agar and add 1 tsp of tapioca starch to the mixture before cooking. (This makes it firmer and stretchier to allow it to set in a jello mold and so that it can be removed without breaking).
Use a silicone mold or spray the inside of a metal mold with a little oil to help it come out after it sets.
Allow it to sit for 2 hours in the refrigerator before taking it out of the mold.
Adding Fruit
If you want the fruit to be suspended in the jello, you need to watch the jello carefully once it starts to cool. After about 10-15 minutes, it will cool enough that it will start to thicken, but will still be liquid. It is at this point that you will want to add fruit or berries to the jello. This way they will stay suspended in the jello and not sink to the bottom.
Note: Don't add pineapple, papaya, or kiwi to the jello. The enzymes in these fruits cause the jello not to set. (You will also want to avoid using these fruit juices for the jello).