If you've had the headache for longer than 72 hours and you have other symptoms (ie a fever, sore eyes, stiff neck etc) please seek urgent medical attention.
If you just have a headache, it could be due to dehydration from not drinking enough fluids or being out in the sun all day. Another common cause is muscle strain, especially around shoulder muscles, which can lead to referred pain in the head. Try drinking a good amount of fluid, taking simple analgesia like paracetamol or ibuprofen. Have a rest and enjoy quiet music in dim light and no heavy lifting.
Other common headache causes include migraines, virus infections like common cold or influenza. Brain tumours are rare and along with meningitis are usually the first things most patients want to exclude. The only way to almost conclusively exclude brain tumours is CT (xray) or MRI (magnetic) scanning. If you have a family history of brain tumours it's worth checking for, though most cases occur randomly.