This is my 2 minute tip guide for beginners.
[This advice is now out of date because the 'Arena Improvements' quest now only grants a 10K reward.]
1) Choose blacksmith and carpenter for your first two craftsmen. That is because one of the early quests for them (Arena improvements) will be easy to finish and will net you a large amount of money ($65K) which will enable you to buy the other two craftsmen.
2) Money management is one of the most important aspects of the game. Simply put, do not overspend! That's how players get bankrupt. Be very thrifty when buying improvements and make sure you have a healthy reserve of cash. My rule of thumb is not to buy an improvement unless I have double the price of it in the bank.
3) The wiki is your friend. Many if not most of the items requested by your customers will not be available in your shop. The wiki will give you the best suggestions. Most of the time, your customers will accept another item in the same research cycle or family tree, so know those backwards and forwards for a quick sale.
4) Pick your friends wisely. Once you buy improvements you will need improvement points. You can either join a guild or trade for them. Pick a guild where the other members are fair and be more than fair when trading so they'll remember you. Don't be afraid to leave a guild if your needs get ignored.
5) Because haggling is a necessity, invest in Silver-tongue as your skill early on. Most of the time you'll need to suggest another item to a customer for one you don't have. It's no use getting tons of customers if they all reject your suggestions.
6) Don't turn down quests as that will close down the whole questline that follows it. If you have the money, give the money for the quests, as the returns far outweigh the costs. If you don't have the money, just keep asking them to come back until you do.
7) Never spend a lot after completing a quest, you may not get the profit back very quickly during the first few. This can lead you to bankruptcy at any level. Treat gold from quests as an increase to your 'base money' ie. how much money you want to have at the end of the day.