This is out of my specility.. but, recently I listened this pod about "Hiring Your First Virtual Assistant: When, Where, and How to Do it Right" & maybe it'll help you.
Link: https://music.youtube.com/podcast/KOqxx8Tzj8E

Here are some steps you can follow to launch a virtual assistance agency :
- Determine the services your agency will offer
- Choose an industry to focus on
- Decide on the structure of your business
- Create a name for your business
- Decide your rates
- Reach out to potential leads and clients
- Develop a marketing strategy
- Use the right tools
- Ask for referrals and testimonials
And also, you may want to check websites like Udemy that offer courses and training on how to start a virtual assistant agency,i hope this was helpful

I’d be happy to assist you! With my extensive experience in founding and advising businesses, particularly in technology integration, I can help you select the right web applications and tools tailored to your needs in administrative tasks and digital marketing. Together, we can develop a solid business strategy to ensure the successful launch of your virtual assistant agency. Let’s discuss your ideas and find the best solutions for your success!