The least effort/risk way to make money when starting is to recruit troops and sell them (you can't sell your starting 100 troops). Just make sure you get a good price, check the strat
buy/
sell section of the market forum
Best way to make money by trading is to buy at a low price (5-10), with low tax. Travel at least 40km from the fief you bought at and sell the goods at a fief with a high price (20-25) and low tax.
You buy goods from the info-tab screen and sell from the inventory-tab screen (type the number of good you want to sell, then scroll to the bottom of the page and click sell)
(WARNING!! Do not click TC! TC stands for town center and you will not get any money when you sell)