Porto is considered the second most important city of Portugal, located in the north of the country, about 3h driving time from Lisbon, the capital.
The easiest way, and most comfortable to reach Porto from Lisbon, is definitely by car, with private transportation.
Please see my article here: https://lisbonguide.org/car-transfer-from-lisbon-to-porto/
But, some guests might prefer to check alternative options for transportation by public bus.
There are usually several buses per day, departing from Lisbon to Porto, usually from Oriente or Sete Rios in Lisbon.
You would need to get transportation to Oriente or Sete Rios, in Lisbon, and then get the bus to Porto on your desired time.
It takes around 3h30h, and makes few stops along the way (usually 2 or 3 stops), depending on the bus company and time schedule.
They are usually 50-seater public bus, operated by Renex, City Express or Rede Expressos.
You can find more information on their website: https://www.rede-expressos.pt/
Hope this information can help, any questions just ask.