We have hadiths that condemn the guitar and that it takes us away from Allah, so I recommend staying away from it. As for singing, it is haram to sing. Anything that’s considered singing (called ghenaa’ in Arabic) is haram (like the entertainment type of singing).
If you want to practice, for your voice, practice not through singing and listening to singers, but by practicing by reciting and listening to Nasheeds for example, or professional Qur’an reciters. As for musical instrument, scholars have stated instruments that are not typically used in entertainment gatherings are ok (for example the piano). But the guitar is generally used in such gatherings and to play haram music, so I would avoid it.