This app was mentioned in 2 comments, with an average of 1.50 upvotes
Start with a friend? Someone who might need an errand service. Offer this for free so it is accepted without resistance. Get a testimonial on how the service was really useful to him/her. At the very least, it makes for a great marketing material.
Furthermore, I would request you to conduct a market research with a large enough sample set broken into demographic, pay-grade, location of residence etc. (things that may affect your ability to identify potential customers).
You can always look for new ideas here, it's a specialized app for entrepreneurs : https://play.google.com/store/apps/details?id=com.devs.bevy
Hoping this was useful :)
Start with setting up Streak on gmail :)
Look for ways to contact these companies that you are targeting. A lot of the times, they are mentioned openly. This isn't sure-hit strategy, but it's free and you can take what you get from this. If your service is something these companies have been desperately looking for, there is a good chance it will proceed.
Consider creating an OpenList - which is essentially an email sign up link submitting to an email list which lets the user know which of their competitors have already signed up/ are on the list. It could make the difference in the process of decision making.
Run highly targeted ads, second to best options after personal emails. Try leveraging the Custom Audiences feature on Facebook. Follow that up with the Lookalike Audience targeting.
More ideas on - https://play.google.com/store/apps/details?id=com.devs.bevy it's a specialized app for entrepreneurs.
Overall consider Inbound Marketing. It's very cost effective. Hoping this helped :)
Cheers!