Младший: used with masculine nouns. Младший брат - younger brother.
Младшая: used with feminine nouns. Младшая сестра - younger sister.
Младшее: used with neuter nouns. Младшее... um... well, the only neuter noun that comes to mind is яблоко. Младшее яблоко, younger apple. Though I'm sure that's not really a thing in Russian. :D
You may consider bookmarking en.OpenRussian.org, since it is an online, open-source dictionary that lets you look up words. It has declensions for nouns and adjectives, conjugations for verbs, and stress marks. It's not perfect / not complete, but it is a good start. See here for the dictionary entry for младший: https://en.openrussian.org/ru/младший. Hope it helps!