The population, as a rule, increases automatically every game month. Check the first item on the news banner that travels across the top of your main screen. That shows your present population and any increase or decrease.
The other way is to spend around 5 gold coins to buy a million population. You should note that not many of the purchased population will necessarily be workers. There will be Children, Housewifes, Invalids and Pensioners amongst the total.
If you just want to produce more workers, then consider disbanding any offensive military units you may have and then deactivate them. The soldiers and officers that made up the units will be transferred back to your workers. Only do this if you do not intend warring, because you have to reactivate the units before you can use them.