This worries a lot of students, but it is fairly easily acquired once you do a lot of performing. Basically, different mics and different sound systems will amplify your voice to different levels. Ideally there will be a proficient person on the sound desk who will set the optimum level based on the singer's voice, the loudness of the band, and the acoustics of the room. If this isn't the case, you may have to judge for yourself, from the audience reaction and any foldback that is relaying your voice back to you. For loud notes and high notes that have intensity, one should tend to pull the mike away or change its angle so you can sing over it, rather than straight into it. For gentle notes, and intimate ballads, you would tend to speak into the mike.

In the recording studio you need to maintain the same distance from the mike at all times, so the sound engineer can have control over amplitude fluctuation.

Body mikes are often used in musical theatre, in which case it's important not to brush over them with hands, clothes or hair during performance.

A mike can't create beauty in a voice; all it does is extend the audibility of the pre-existing qualities. The main challenge is to develop a great sound that people want to hear - using a mike will come easily with a little practice.