This might seem a little random, but how would I know if there is a ghost in the house, or if it is just in my head?

Ghosts are not real, so probably just in your head.

Then how are there sightings?

It's mostly stuff in people's head, or exaggerating normal things such as wind etc. They don't see a ghost, they see something that they interpret as a ghost. The brain can get confused, especially in times of grief or stress.

Despite interest in ghosts for many hundreds of years, there is really no evidence that they are real.