atarasii gakki wo hikuno ha muzukasii desu ga, mainiti rensyuusuru to zisin ga demasu.

In Japanese, when you want to talk about an action as a “thing” or topic, you nominalize the verb by adding の (or こと). Here, 弾くのは literally means “as for playing (the instrument)…” so you can then comment on that action (難しいです).

The particle は marks the nominalized phrase 弾くの as the topic of the sentence. In English you might think of it as “When it comes to playing a new instrument…,” but Japanese uses ～のは + は to set that phrase apart as what you’re talking about.

Japanese が has multiple functions. Here, the first が is a conjunction meaning “but/although.” It links the two clauses:
• 新しい楽器を弾くのは難しいです (Playing a new instrument is difficult)
• 毎日練習すると自信が出ます (If you practice every day, you’ll gain confidence)
It’s not the same が that marks subjects (like the second が before 出ます).

The particle と after a verb indicates a natural or inevitable result: “when/if you do X, Y happens.” Here, 毎日練習すると means “if/when (you) practice every day, (then) …,” implying that confidence automatically follows from daily practice.

毎日 is a time-expression functioning like an adverb (“every day”). In Japanese, many time-adverbs (毎日, 毎週, いつも, たくさん, etc.) can directly modify the verb without a particle. You don’t need に with 毎日 here—just place it before 練習する.

Both are possible, but the nuance differs:
• 自信がある simply states “I have confidence.”
• 自信が出る (“confidence comes out/emerges”) emphasizes the process of gaining confidence. It suggests that confidence grows or appears as a result of practice.