Step 1 : Obtaining your Starting Capital
There are many different ways for you to obtain your starting capital. You can sell raid runs, this can be your main source of income even long-term, doesn't have to be just your starting capital, if you enjoy it do it.
Another way would be selling BOE's that you obtain through running BFA raids. The BOE's don't sell for as much as they used to but it is still a pretty good side income if you happen to get a lot, really depends on RNG.
In my opinion the best way to obtaining your starting capital would be through gathering farms. You can pickup the professions herbalism/mining and just go out and gather. You can use the website the undermine journal to figure out what herbs/ores will be selling for a good price and then you have to experiment by going out and gathering that herb/ore and seeing if it will sell on your server.
My personal favorite materials to gather would be (DISCLAIMER** the prices of these will vary based on your server of course, Ghost iron ore could make me the most gold on my server, but on your server it could make you the least amount of gold):
- Ghost iron ore/trillium in Pandaria
- Whiptail in Uldum
- Azhara's Veil in Tol Barad
- Cobalt ore/saronite/titanium in Northrend
- Gathering BFA mats that sell for good on your realm.
If you don't enjoy herbalism/mining I would recommend just go farm some Volatiles, Primals or Eternals as these all sell pretty well.