Welcome to the daily halacha (Jewish law). Are you allowed to use musical instruments like guitars during tefillah (prayer) during the week? Obviously, we're not discussing Shabbat, because you're not allowed to touch musical instruments on Shabbat. It's totally muktzeh (untouchable) on Shabbat.
During the week, at Shacharit, at Hallel—let's say, on Rosh Chodesh—can a person use musical instruments? This is really a long discussion in the Hachamim (sages). I'm just going to quote the words of Hacham Ovadia Yosef's son, Hacham Avraham Yosef: A person should refrain. Nobody ever in Jewish history, says Hacham Avraham Yosef, brought instruments in the middle of tefillah, in the middle of prayer, during the week in order to enhance the tefillah. It is not our tradition, and we never did it. We never saw anybody do it. We didn't see our fathers do it, our grandparents, our Hachamim.
Therefore, a person should not use musical instruments in tefillah during the week. Obviously, if you want to sing, you should sing, whether it's during the week or Shabbat. But we never saw people use musical instruments in tefillah in the Jewish communities. We never saw our rabbis do it. Therefore, a person should always avoid using musical instruments during the week at tefillah for Shacharit.