Should I reveal the items effect?

Hi all, I'm a new dm and I don't understand in what circumstances I should disclose the effect of a new magic item I'm giving my player and when keep the mystery around it. For example if I give my player an amulet that shines when zombies are close, do I tell them what it does or let them discover it for themselves? I fear that If I don't tell them they could sell it at the first occasion without knowing, or they will keep it in the bag and never see it shine. For the context we are all new to dnd, characters lower with lvls lower than 4.