One of the most simple ways to keep a steady stream of income coming in on the side is to get the transmute spell early on and to purchase iron ore from every merchant you visit.
The transmute spell will change iron ore to silver ore the first time it is cast, and then the silver ore to gold ore the second time it is cast.
Take your gold ore to a blacksmith with a smelter to turn it into gold ingots, then use the blacksmith forge to turn each gold ingot into 2 gold rings. Once you have a fair amount of gold rings find a local beggar and give them a gold coin which will give you the blessing Gift of Charity, increasing your speech skill so you can get a better selling price for your items. Gift of Charity lasts 1 hour of real time which is more than enough time to go to a merchant that buys jewelry to sell all your gold rings.
This also has the side benefit of increasing your smithing when you make the rings. Smithing is a core skill for most players as it enables you to significantly improve your weapons and armor.
NEVER sell jewels/semi-precious stones on their own. When you use them to make jewelry at the blacksmith forge with your gold or silver ingots, the return on value when you sell them is significantly better and, again, it increases your smithing skill.