"Yes, I am a boy."
Translation:Oui, je suis un garçon.
If you have a PC you can switch your keyboard to an international one. Easiest solution for me. For Macs, I believe you just hold down the key and the accented options appear. Also on the web version of Duolingo is a selection of accented characters you can choose from right below the exercise box.
Lastly, on a mobile device you can just press and hold down the key for the corresponding accents.
Nous describing professions, nationalities, and religions can act as adjectives in French when there isn't a second adjective. When this particular class of nouns are acting as adjectives you don't use an article or determiner.
Je suis médecin. BUT Je suis un bon médecin.
Il est avocat. BUT C'est un (mauvais) avocat.
Garçon, femme, homme, etc. do not fit in this special case and always require a determiner in front.