A few foods that start with the letter "J" are jalapeno peppers, jicama, Jerusalem artichokes, juniper berries and Jello. It's important to note the the "J" actually makes an "H" sound at the beginning of "jalapeno" and "jicama," as both words originate from the Spanish language.
Jalapeno peppers are mild to severely hot peppers often used in Mexican food. To lessen the heat of a jalapeno, cooks can remove the seeds and membranes from inside, as they are the hottest part of the pepper. Jicama, on the other hand, is a root vegetable. It is crunchy and lightly sweet and is often peeled and eaten raw in salads or with a dip. The Jerusalem artichoke is also a root; according the the BBC Good Food page, it is a specific kind of sunflower. Cooks can prepare it much like a potato. Juniper berries are most often dried and used as a spice, such as to flavor a roast; they're also used as the main flavor in gin. Finally, Jello is not a vegetable or fruit at all, but instead, it is a brand name of flavored gelatin.