If you ever decide to start out fresh on a new server, for any reason, you'll quickly notice that you need money, asap. When there is no sugardaddy main to send you some hundred g in starting funds it might actually even happen that you can't afford your very first skills (this has happened to me)!
Although collecting money is quite easy, you might need some kind of plan when going about doing it, to be able to collect enough for some very necessary expenses in your way - level 20 mount, level 40 epic mount, all your skills, any imba gear you might find at AH and etc. Just one Frostweave bag could easily cost you 50-60g, which is no peanut money for a lowbie.
This won't be one of those "this is how to make huge amounts of gold in 24 hours" kind of guides, but just some easy to follow thumb rules to keep the cash flowing in a nice and steady pace. It might not make you ridiculously rich, but at least you won't have to turn into one of those sad to look at beggars, "can I has 2g for gear plx".
The only thing you ever really have to keep in mind to be able to make some money is - everything is worth money. In WoW, any kind of effort put into killing or collecting items will produce some amount of money. In WoW time definitely is money. The gold farming guides will tell you the absolutely most profitable way to spend this time, but all you really have to know is that you will make money whatever you do.
Thumb rule number one is only true if you follow thumb rule number two. Save everything you find. It isn't worth anything if you don't collect it and sell it to someone, be it a vendor or a player. Killing a mob and not looting it isn't worth anything. When you're still a lowbie even any gray is good to keep, although I recommend keeping all the grays you can carry at any level. An empty bag slot isn't good for anything. Cram your bags! Is thumb rule number two.
Although grays are a good way to get started on your money making, it won't get you much money later on. The best way to make a fast buck is by collecting "whites", that is any item used in a profession. The fact that an item is white and not gray indicates that it is used in some profession, so it is really easy to distinguish whether an item is good to keep or not. If you have to choose between a gray and a white, the white will usually turn out to be worth more money. The white does come with a little more hazzle though, since you won't earn anything from it just by vendoring it. You have to place it on the AH.
Whites can either be collected by killing mobs (meats for cooking), or by getting a gathering profession such as skinning, herbalism or mining. Although all these are really profitable at low levels (and later on of course), you can check the AH to see which one makes the most money on your particular server. I have seen servers that sell stacks of leather for one tenth of another server, so knowing the status of your servers market for a particular type of goods is good before deciding which profession to get.
Crafting professions aren't much profitable until way later on in the levels. Actually crafting professions usually cost more money than they earn you, especially while you're skilling it. By selling meat, herbs, ores and/or skins, you're the one earning a buck from someone else skilling their craft! Since it is way easier for a level 80 to earn money by doing dailies and such, they usually don't feel their time is worth grinding mats for. So they will pay you to do it for them!
Unless you feel a desperate need to craft your own items, I strongly recommend collecting stuff for others to use. It is a great way to make some quick bucks!
Save all greens! Yet again with the saving. Up until level 30-40ish you can easily get about 1g+ for any green you find, no matter what stat it has. It's also usually not worth disenchanting until later levels (40+) so if you get the option in an instance, I would generally recommend to choose not to disenchant an item if it's below level 40. I do not know who wants to buy all these greens, but they always sell!
When finishing a quest and turning it in you often get to choose between some different rewards. Unless you're going to pick one because you actually need it, you should always check which one is worth the most to vendor. Sometimes the difference is several g!
To save you some time while questing, and not have to interrupt it continously because your bags are full and you have to go to AH to empty them, I also recommend getting a bank alt. Just any char, standing ready at the AH and postbox in any city who can do the job for you (or rather, your char) so that you can keep on questing instead. That way you can just "empty" your bag at any postbox instead, by sending it to your bank alt. Whenever you feel like it (although more than once a month) you log over to your bank alt and do the money earning business through that char instead. Saves alot of boring travel time.
On lower levels where you'll ding quite fast, and thus visit a city most of the time anyway, this is less of an issue. But later on, especially when travelling far away like Outlands and Northrend, this will become really handy.
But my item won't sell!
Don't be discouraged if your item comes back to you in the mail. There are three reasons this could happen. Most likely, you've overpriced it. This is usually easy to avoid by checking the current prices of a particular item before placing it (or getting an addon that does it for you, I don't use that however). Undercut reasonably and it will most likely sell. If you're the most expensive option, you probably won't sell. If there is no other item like yours already at AH, you can decide the price for yourself, but it comes with the risk of being undercut. It's something you have to accept.
The second reason your item didn't sell could be pure bad luck. No item is worth anything unless people want it. Supply and demand and all that. If you're really unlucky, even items that usually sell really well, don't sell just because no one needed that item during the time it was at the AH. It is rare, but it does happen.
The third reason is that your item simply isn't worth anything. Sometimes you find some whites and greens that just never sell, because the demand for them is so low, or the supply of them too high. If your item return to you for a third time, this is probably the reason and you can vendor it instead.
All these are some simple rules to think about when leveling that lonely little char. If you stick to these rules I can promise you that you 'll be able to afford just about any expense coming your way on your path to level 80.