you could say "ぼくはおとこのこです" (boku wa otokonoko desu), which means "I am a boy". I think you could also use "わたし" (watashi), but I normally hear the male characters in anime use boku, so I think the first option would be the best phrase to use.

The sentences no one ever says are ぼくはおとこのひとです、ぼくはおとこのこです You can use わたし for both male and female.

Yes, you can say "I am a boy." in several different ways in fact. As Kendra pointed out, anime characters say "僕（ぼく）" a lot, which is correct, but is usually used with informal speech. Depending on who you're talking to and your situation, the sentence can look very different. A couple of examples: 私は男の子です。（わたしはおとこのこです。） - from my understanding, this is the most correct way to say this sentence in です form. 僕・俺は男の子だ。（ぼく・おれはおとこのこだ。） - this is what you'll probably hear in anime. In this particular case, though, 俺（おれ） would be a very weird sentence, not because it's not correct, but because the people who use 俺 are usually older men, not boys. 僕 is usually used by younger men and boys alike. You can also do this if you're really into talking too politely: 私は男の子でございます（わたくしはおとこのこでございます。） This, at least to me, sounds incredibly forced, but there might be differing opinions on the matter. Don't use this unless you have a situation in which you have to speak with some sort of CEO or something. Hope I'm helpful, and have fun studying Japanese!