What does “repent” really mean?

What does repent actually mean in Acts 2:38? I go to three churches, so do I need to tithe to all of them? What does Jesus mean when He says in Matthew 17 that they don’t have mustard-seed-sized faith? My wife wants to divorce me. What should I do? repent, Acts 2, tithing, Matthew 17, faith, mustard seed, divorce, counseling