Singing out of tune may be due to you trying to sing in a key which doesn't suit the range of your voice, i.e. the key in which somebody else sings a song will be chosen to suit their voice, but will not necessarily be one which you can sing in. You may have better luck in a lower or higher key.
Alternatively, people go out of tune due to poor breath control, often due to the lack of breathing technique mentioned in my previous posting. If your breathing is restricted, you are less able to control the note which emerges, and it is more likely to go out of tune.

Volume is another issue. Some singers do not sing particularly loudly. They encourage the audience to quieten down to a level where the song can be heard. With some audiences this will not happen.
Quack!!
Geoff.