The word Judo is neutraltherefore the correct article is in the basic form the.
In the German language, the gender of each noun (in the lexicon) is fixed.
German nouns belong either to the gender masculine (male, standard gender) with the definite article der, to the feminine (feminine) with the definite article die, or to the neuter (neuter) with the definite article das.
for masculine: points of the compass, weather conditions (east, monsoon, storm; but: the thunderstorm), spirits (vodka, wine, cognac), minerals, rocks (marble, quartz, granite, diamond);
for feminine: ships and planes (the Deutschland, the Boeing; but: the Airbus), brands of cigarettes (Camel, Marlboro), many types of trees and plants (oak, poplar, pine; but: the lilac), numbers (one, million; but: the dozen), most inland rivers (Elbe, Oder, Donau; but: the Rhine), …;
for neuters: cafés, hotels, cinemas (the Mariott, the Cinemaxx), chemical elements (helium, arsenic; but: the sulphur, masculine elements on -stoff), letters, notes, languages and colors (the orange, the A, the English), certain brand names for detergents and cleaning products (Ariel, Persil), continents, countries (the articleless ones: (old) Europe; but: Lebanon, Switzerland…).
How does the declension of Judo? Here you will find all forms in singular and plural:
[1] She has been training in judo for self-defense since she was fifteen.
