First of all, congratulations on your decision to start your own digital advertising agency.
As you probably already know, the biggest challenge with such a business model is finding clients.
I suggest that you be clear on the kind of client you are shooting for.
Once you have a clear idea of what type of business you'll appeal to, come up with buyer personas.
Many people who go into the digital advertising agency business overlook this, and that's why they fail.
Figure out where these personas hang out online.
What are their needs? What content do they usually gravitate towards?
Once you have a good understanding of this, you can create content and plan your advertising campaigns accordingly.
A great place to start is LinkedIn.
One shortcut to all of this is finding an agency you'd like to become and reverse-engineer them.
Figure out where they hang out, understand their content, and mimic what they do.
Of course, it will be ham-fisted and rough around the edges, but it gives you a headstart, and pretty soon, you can fine-tune and adjust your message until you get solid results.
Is it going to happen overnight? Of course, not.
You have to be patient, and most importantly, you have to be willing to learn from your mistakes.
Congratulations on your journey.