Practice as much as possible. You should know your songs and feel comfortable singing them. Learn to hit the correct notes and know the words to the songs. Record yourself if that helps you find areas in which you can improve.
Sing in front of friends and family. This is a simple way to help you become accustomed to singing in front of others. Give a performance of one or two songs. Sing your heart out. Ask for feedback. Invite more friends as you become more comfortable.
Use visualization methods. You have to believe in yourself so that you can sing in front of people. Visualize yourself singing your heart out. See the audience's faces. See the smiles and hear the applause. Imagine yourself hitting the difficult notes. Do visualization exercises every day.
Practice voice warmups. These exercises are essential to help you sing at your best capacity. You can try something such as breathing out through your mouth, while releasing all of the air in your lungs. Then relax your stomach muscles and open your mouth. Let the air back in. Do this three times. Then add to your warmup. The next time you release the air, make an "sss" sound. Try this again, using "ssh" and "fff" sounds. These help build vocal muscles, which will build your confidence. Consider books to help improve your singing.
Participate in a karaoke event. Many bars have karaoke nights, so take advantage and practice your singing in front of others. Pick a song that you know really well and get up and make the song your own.