The conjugate base of NH3 is NH2, which has a charge of negative 1. When NH3 acts as an acid, it forms its conjugate base by donating a proton. This removes a hydrogen atom and lowers the charge by one.
When finding the conjugate acid or conjugate base of a molecule, it is important to consider how acids and bases react. An acid donates a proton, whereas a base receives a proton. Since a proton has a charge of positive 1, an acid that loses a proton becomes more negative when it becomes a conjugate base. Similarly, a base that gains a proton becomes more positive when it becomes a conjugate acid.