Pick a pitch you can easily refer to – like a Middle C if you’re often near a piano, or the A440 of a standard tuning fork (440Hz).
Then, frequently play this pitch through the course of each day.
After doing this for a while start trying to hear the pitch in your head and sing it before you play it. Check whether you were right.
The more you practice, the more reliable your ability to reproduce your reference pitch will become.
This kind of “tuning fork ear training” is a good first step towards developing absolute pitch.
Once you have one reference pitch internalised you can start addin