Logo & branding are the key pillars of a company's identity. The logo should be able to be recognized by people.
It does not matter whether the logo is simple or not. The important thing is it should be solid enough for recognition.
A few things could be considered for an advertising logo:
- Initials (You could use the company initials as the logo)
- Types of services (If you provide billboard advertising, a flat billboard could be included in the logo)
- Play of words (Adding creativity to your logo)
- Colors (Choosing one or two main colors)
For more inspiration and ideas, you could look around on Dribbble