Great idea! I'm currently in the process of launching two online businesses.
This is the process I've gone through so far:
1. Decide what business you want to launch - product or service business. Either option has pros and cons depending on the type of product or service.
2. Define your niche and target audience. Where do they hang out online, what do they do, why would they use your product or service.
3. Join Facebook Groups and Discord channels, Reddit and Quora threads with discussions around your niche and target audience and start engaging and answering questions and providing recommendations..
4. Launch a blog or YouTube channel and start documenting your journey about the online business you are launching and the product or service you have in mind.
5. Offer a free trial of your service or sample of your product to capture email addresses and start building an email list of potential future clients.
6. Once you have an MVP of your product or service, launch and get feedback from your audience about changes and improvements to your product or service. Build up testimonials as social proof.
For products, if you are offering physical products you want to have a number of sales channels like Amazon, eBay etc and you need to consider sourcing suppliers and drop shipping etc which involves quite a few moving parts.
If you want a good template to follow for an online business, read Tim Ferris 4 Hour Workweek - packed full of nuggets and has helped with my online businesses.
Happy to have a chat and discuss further if you want a deeper dive on this.