Hi! The great thing about being an entrepreneur is that there is no wrong (I mean, ethically there might be, not talking about that here :D ). How creative can you get aquiring business?
Paid ads are a great way to get quick and (in the beginning) cold traffic. After a little while you can start retargeting these people to increase conversions. It might be good to know that the higher the conversion value, the more money you can spend to aquire new customers. So upsels / order bumps help with creating a bigger budget for advertising.
The 'what' I will let you decide yourself, but since you're looking to build a service based business, I would not be a good fan if I didn't recommend the following books:
DotCom Secrets & 30Days books from Russell Brunson (other books as well, but these are a great starting point).
$100M Offers & Leads from Alex Hormozi.
The books are not tied to 'services', though are perfect if you're starting out with the idea of actually providing value to your customers.