Don't put all your eggs in one basket. It's a lot better to spend your working capital on buying forty items for auction, each with the potential for profit, than to take all your working capital and invest it in that one purple leatherworking recipe that you hope will make you several hundred gold. If that puppy doesn't sell, or doesn't sell for what you want, you've just wasted all your working money, and deprived yourself of a lot of flexibility. Leave speculating on purple items until you have a few thousand gold squirreled away.
There are a lot of skills and spells you can train as you progress, each of which costs money. When you can afford to, you should train all the abilities that your class trainer offers. If you're completely broke, it's fine to put off upgrading abilities you rarely use for a level or two so that you can upgrade your most-used abilities. If this happens, you should ask yourself whether you are spending too much money on buying unnecessary equipment upgrades or leveling production trade skills. Be sure to save enough for class abilities and riding training before spending money on other things. This assumes you're earning money at a rate where buying skills makes a difference at all. If you have a hundred gold from two gathering professions by level 20, those skill ranks costing a fraction of a gold won't make any difference to your purse.
If you are dead set on leveling a production trade skill instead of taking two of the gathering skills, remember that not everything your trainer offers is worth buying. While it might be nice to have a long list of colorful shirts and dresses to produce as a tailor, for instance, the truth is they offer very little in the way of potential revenue. Also keep in mind that, generally, whatever items you craft at lower levels will not likely sell for more money than you could have made by simply selling the raw materials used to make them. For this reason, two gathering skills are highly recommended until you get closer to 80 (70 if you don't have the WotLK expansion or 60 if you don't have the TBC expansion).
This was a farming spot that I made vastly popular on YouTube sometime last year. It is my FAVORITE farming spot of that era, simply becuase it was an old world farming spot that allowed you to hit almost 3 different markets. With this spot you can get cloths: Felcloth, runecloth, and mageweave. Herbs: Goldthorn, ghost mushroom, goldclover, and fadeleaf. World drops: rich purple shirt. AND a plethora of transmog from a single run. I literally had some pulls where I came out with 17,000g in item value for 10 minutes of farming. It was insane – actually it still is. Anyway If you have a druid or monk, then this is an awesome farm to hit ESPECIALLY if you are on a high populated server. Generally just pull the entire instance, and circle it back around the front. Mega gold to be made here.