First power level any skill. The easiest is sneak. Don't go to sleep and level up!

Once your skill is at 100 look for a mercantile trainer.

Basic: Foroch at the Gottshaw Inn southwest of Kvatch. Mach-Na(Owns a store with her name) in Cheydinhal.
Medium-level: Margarte (Involved in Fighters Guild). Seed-Neeus who owns Northern Goods and Trade in Chorrol.
Master: Palonirya who runs Divine Elegance in the Imperial City (you need 10,000 septims for her to agree to train you).
After you find a trainer, train with them 5 times. Then sleep to level up and go back and train 5 times again. And just repeat.

If your Mercantile doesn't reach 100 then power level another skill and do this over.

You may need to use a duplication glitch to earn money. But there are plenty of methods on this site already =).