By "a natural sounding way ", I suppose you mean "Japanse-Japanese（日本人が現実場面で話す日本語）" or "Not-English-Translation-like Japanse(英語の翻訳臭くない日本語）”. >for example if you have a choice between 2 cakes and you wanted your friend to pick which one they want? Let's assume here the name of your friend be "よしこ(Yoshiko)". Then I suppose one of the options you could say would be: "あ、好きな方、取ってね“ or "あ、よしこちゃん、好きな方（ほう）、取って (Oh, you pick whichever you like)” ※ I believe Japanese girls never call their friends "あなた“. I personally cannot remember the last time I heard and spoke the word "あなた” in my real, everyday life,

You don't have to be sorry. "It's up to you" is usually translated as "あなた次第です。", but in your example, you can also say, "あたたが選んでいいですよ。or あなたが決めていいですよ。(It's kind of "You decide.")" I hope it helps you^^

>I was making plans with my friend and wanted to ask her like “if it works for you we could meet next Tuesday?” what would be the best way to say that? Let's suppose you be two of girls and think it feels comfortable to speak a bit decently even among friends (because we are girls). Then one of your options would be: 「もしよかったら、今度の木曜に、会わない？」

One more question! If, for example, I was making plans with my friend and wanted to ask her like “if it works for you we could meet next Tuesday?” what would be the best way to say that? I know how to say 都合がよかったら, but I think it might be too formal to use with a friend?