Do breathing exercises. By doing breathing exercises, you will not feel as exasperated when singing. For example, try breathing in for 10 seconds and then breathing out for 10 seconds. Repeat this a few times until you feel comfortable.
Make sure that you are drinking enough water. When you are singing, it is always important that you have water. The drier your throat becomes, the more you will tense up while you are singing.
Check your posture. Bad posture can cause you to force the words out, which will make you tense. Having good posture means standing up straight with your shoulders back.
Practice some basic scales. Doing scales will help you to warm up. By warming up, you will be able to practice making sounds that you will be singing in your song. It will help to relax your voice and prevent you from tensing.
Know which places to breathe while you are singing the song. It is generally not a good idea to breath before the end of a phrase. For example, if there is a comma after a group of words, then breath after that, but not before.