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 adding to it with other pitches, and combining it with your sense of relative pitch to accomplish other musical tasks.
You’ll also find that this kind of focused listening and awareness of pitch and frequency improves your overall musical sensitivity and aural skills.