I find one of the most frustrating things to do when playing the piano for congregational singing is the introduction.
Beginning the song with the right tempo and just enough of a tune to help the singer’s tune into the song to be sung has always been a bit of a struggle with me.
The easiest way to make an introduction is to take the last few measures (one to four) of the song that you are playing and make that your introduction.
Play that phrase in the same tempo and style that the song will be sung in.
Sometimes in order to let the congregation know that it is time to commence singing end the phrase with the chord of the fifth note of the scale your music is in played as a seventh (7th). The sound of this chord while not a suspended sound still alerts the singers that something is coming next. It give the feeling of saying “Oh!” such as “Oh What a Friend We Have in Jesus”. While that first word “Oh” is not the first word of the music if you were to play a 7th chord right before the singing  it would convey that message.
A seventh chord before the singing is not always an appropriate sound and you will need to use your ears and best judgement as to when to include one.

This works quite nicely when you are first starting to play accompaniment piano but as your skill at the piano increases you may find yourself wanting to do a bit more for an introduction.

Some Tips:

• Listen to how other pianists begin songs and see if you can figure out the chord progressions they are using and how maybe you can borrow from that.

• Don’t get too fancy or too busy with your introductions. You are attempting to accompany not drown out.

• When practicing a song practice the introduction too and make notes in your music if you play with sheet music or a lead sheet.

• Make sure your introduction is evident and don’t ‘suprise’ your singers with a new one if it is different from what you normally do. Play it for a few Sundays during times such as offertory, prelude, etc.