下 (noun) means “below”, “down” or “under”.
Noun を向く means “to face or turn towards something”.
So 下を向く means “to look down”.
んじゃねぇ is the negative imperative (don’t!), used with a verb dictionary form.
It’s the broken version of んじゃない which is commonly used in conversation. And it’s also the broken version of のではない which is rarely used as it sounds too stiff. んじゃねぇ sounds very rough and aggressive so It’s normally used in casual male speech.
So 下を向くんじゃねぇ means “don’t you dare look down”.
Main Clause: バレーはスポーツだ
バレー means “volleyball”.
Please note that “ballet” is also called バレエ in Japanese. When we write it, we use the katakana エ for ballet instead of the prolonged sound mark (ー) in order to distinguish between them. But the pronunciation is the same so you usually need context in speech. If you want to make sure, you can say バレーボール for volleyball and バレエダンス (ballet dance) for ballet.
は is the particle indicating the topic of the sentence.
スポーツ means “sport”.
だ is used to make the statement affirmative and declarative.
So バレーはスポーツだ means “volleyball is a sport”.
Subordinate Clause: 常に上を向く
常に means “always”.
上 means “above”, “up” or “upon” so 上を向く means “to look up”.
The subordinate clause is inserted before the noun to add more information. So バレーは常に上を向くスポーツだ means “Volleyball is a sport where you’re always looking up!”
So when you want to modify a noun with a clause in Japanese, you don’t need a relative pronoun such as “where”, “which” or “that”. Instead, you put the subordinate clause before the noun that you want to modify.