First, start by warming up. Research online for voice warm-ups you can practice before you even attempt to sing a real song.
Next, sign up in a choir. There are plenty of free choirs that you can join and practice with. This is a great opportunity to learn new techniques and it is free too! See if there are any organizations in your community or ask to join your church's choir team.
Sing often. Even if you are just singing in the shower or car, every time you sing, it will help.
Listen to your voice. Record yourself singing and see if you can pick up on things that you can work on. Whether it is your tone, or the rhythm see what you need to practice on and work hard on that.
Most importantly, believe in yourself. Having faith and self-confidence in yourself will help you become a better singer, since facing an audience with confidence will make you stronger and less nervous.