If your intended fruit (or fruit blend) is low in pectin, you will either need to add pectin or add a fruit high in pectin (e.g. apples, grapes, etc.).
Commercial pectin is available in both liquid and powdered forms. There is special type of pectin (low-methoxyl pectin activated by calcium) which does not rely upon that same balance which permits jams/jellies to be made with less, little, no sugar, or the use of other sweeteners (e.g. honey, Splenda®, etc.). Regardless of the type of added pectin, follow the recipe and instructions provided in the package. Keep in mind that a "full rolling boil" is one where it cannot be stirred down no matter how hard you stir.
The gel point is 8°F/4°C above boiling point of water. Make a note of the boiling point of water with your thermometer before EACH jelly/jam making session; then test to the incremented temperature. THIS TEST IS ONLY GOOD IF pot is being stirred vigorously, thermometer tip is not resting of pan bottom, and there is no accumulation on thermometer tip.
If you do not have a thermometer:
Place several spoons into freezer before beginning jamming session. When you think it is close (changes to viscosity and color occur), remove pan from heat, place a small amount on one of the spoons (NOT by dipping the cold spoon) and place spoon back in freezer. After a moment or two, check to see if the spoon feels room temperature on the bottom: then, if so hold the spoon vertical. The setting point has been reached when the jam/jelly does not run; or, for a softer set, it moves very slowly. If not set, place the pan back on the heat and boil for another 5 minutes and test again. [from The Blue Chair Jam Cookbook by Rachel Saunders]
There are other methods of testing without a thermometer, but this is the one I have found easiest.
Jar and process following safe and approved methods; OR place in refrigerator and consume within 3 weeks.