Не беспокойся, я найду ключ.

Both mean “to worry,” but:
• беспокоиться is more “general worrying,” often about someone or something’s state.
• волноваться can also mean “to be nervous” or “to get excited” (e.g. before a performance).
They’re interchangeable in many contexts. Colloquial alternatives include не переживай (“don’t overthink it”) or не парься (“don’t sweat it”).

Russian verbs show person and number in their endings, so you often drop subject pronouns. Найду already means “I will find.” Including я adds emphasis or clarity:
• With pronoun: Не беспокойся, я найду ключ.
• Without: Не беспокойся, найду ключ. Both are correct.

Russian expresses future tense differently depending on aspect:
– Найти is perfective (“to find” as a completed action). Its simple future is (я) найду … (“I will find …”).
– Находить is imperfective (“to find” as a process/habit). Its future is formed periphrastically: я буду находить … (“I will be finding …”).
You cannot say я буду найти; a perfective verb doesn’t need “буду” to form the future.

Ключ is a masculine inanimate noun. In Russian, inanimate masculine nouns have the same form for nominative and accusative. So:
• Nominative: ключ (subject)
• Accusative: ключ (direct object)

• беспокойся – stress on the third syllable: бес-па-КО́Й-ся (беспоко́йся).
• найду – stress on the second syllable: най-ДУ́ (найду́).

Yes. You’d use the accusative pronoun его for a masculine noun:
Не беспокойся, я его найду.
Here его = “it,” placed before the verb (я его найду).

Besides Не беспокойся, you can say:
• Не волнуйся – literally “don’t get nervous.”
• Не переживай – “don’t overthink it” or “don’t fret.”
• Не парься – very informal “don’t sweat it” or “no big deal.”