How can I learn to play piano by ear, especially with chords and contemporary worship music? I m not very good at reading sheet music?

Ask the pastor if he has any suggestions. You might be able to pick music that reflects his sermon topic, for instance, or something they are celebrating in the church. If they are having a special service for children, that would point you to one kind of music. A memorial service for someone who died would be something else. For someone who died in Iraq, still different music. Browse around the church. Is there a hymnal they have used in the past? Start with music from that source, because it is music with which people are apt to be familiar. As you get to know the congregation and the service, you'll be able to feel more confident in making your own choices. Don't be hesitant to ask for suggestions from older members of the community at the church. They'll be flattered to be asked and that will help put them on your side.

Learn reading sheet music, it makes much more sense