Tell them truthfully that you were looking for a job suitable for your skillset and you found it on a career website, job board while searching for jobs on LinkedIn, etc.
A friend, colleague, or family member suggested about this company and the job opening they have that fits your skill set.
A colleague or friend who is an ex-employee of the company informed you about it after they heard about the job opening, and suggested to apply.
You read about this job in a press release, article, or news source.