Breathing and Pickup Beats
When the pianist plays the introduction, the congregation knows you are about to start the song. There are two ways you signal to them that is time to sing.
Gesture.
Breath.
The one the congregation is aware of is your gesture. They are probably not aware of your more powerful signal: the breath.
You should already breathe naturally while you song lead. After all, if you are about to sing with the congregation, you probably just took a breath. The adjustment, then, is to make sure they can see that you have taken the breath.
So lift your head and open your mouth!
When you breathe, you should also increase the size of your gesture. This means that even though the most important beat is the first beat, in a standard 4/4 hymn, the largest gesture will be on beat 4. This beat right before the congregation sings is called a preparation beat. Clearly signaling the prep beat is the single most important part of song leading.
There are many hymns that have one extra beat, called a pickup beat, before the barline. You can identify these hymns easily because there will be only a word or two before the first barline. Although this might appear to be an extra measure, the beat is actually borrowed from the end of the hymn. If you add up the beat in the first measure with the three beats in the last measure, you have one complete measure of four beats. This also tells us that the hymn begins on beat 4. That means your all-important prep beat has moved back one beat, to beat 3. Any time you have a single beat pickup beat, the prep beat will move back one beat. You should still signal to the congregation that the next verse is starting by increasing the size of the gesture and showing your breath on the prep beat.