Not Good At
Noun, な adjective, and の adjective
みじゅく
俺も昔は未熟で好き勝手してたよ。
I used to be immature and I did whatever I wanted too.
Not yet ripened, means you haven't really come into form yet. It means you're not yet good at something.
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.