Enter join one or all of the skilling/questing/minigame etc markets. Either try to find one which isn't heavily saturated or undercut everyone else's prices so you gain customers.
Keep that going until you have a decent reputation and look at employing workers. Possibly expand from there into gp swapping (keep your profit margins low and don't get caught out by rates changing). Bond sales are also easy vouches, but anyone that isn't a newcomer will discard them when it comes to checking your legitimacy.
Then once all of that is done, you could attempt to move into the gold buying/selling market. It's heavily saturated so you'd probably not get very far without a solid reputation.
That's a typical route people take here on Sythe. There are alternative markets for RSPS's and CS:GO. If you know much about those you could try getting into them too. Maybe start trading in a developing RSPS and hope that it grows from there, giving you a head-start on competitors.