First, lets start with leveling. Leveling a JC can be a costly process, especially if you attempt to do it all at once. The problem you run into, as with any profession now days, is that the mats from the old zones can be costly due to low supply in the AH. So you have 2 choices, either spend the money or farm. Neither is all that appealing. I suggest buying where its cheap and farming where it is not but if you are in a rush you can just spend the money. You can also pair JC with mining if you are leveling a new toon so that you gather the mats as you go. This is also nice because you will be spacing out the leveling process and not eating that huge cost all at once. You may even get to use some of those items you make. If you already have a miner you can still pick it up and drop it for another profession later. This will allow you to stay focused on this character and not have to log to your other character just for mining.
Now, believe it or not there is money to be made during leveling. There are two things you should be doing to make money. First, try to sell at least some of the items you make. Most people use http://www.wow-professions.com/ for there leveling. This site will walk you through what mats you need and what to craft to level quickly, efficiently, and hopefully cheaply. I would always try to sell 2 or 3 of an item that I craft. The rest I would send to an enchanter alt to DE. If you don't have an enchanter it could be a good time to start one as they will be getting lots of mats from DEing the items you made. You can create enchanting scrolls and sell them or just sell the enchanting mats. Both will make you some money and offset at least some of your costs.
The second and more important thing you can do to make gold is more long term. While leveling notice what mats are in short supply for your leveling process and that you need a lot of to level. Two examples of this are Moss Agates and Hessonite. You need several of both of these and both seem in short supply. Notate this while you level. These are niche markets you can get into later and make some real money. Often I will buy Tin Ore for 20g/stack or less and prospect it. If I get at least 1 Moss Agate I can sell it for 25g/each or more! I usually end up with several. Hessonite, which is prospected from Obsidium Ore and Elementium Ore, I can sell for 75g/each. This little process holds true for all professions.
As you get more leveled up in JC and start making gems instead of jewelery you can sell many of these gems to twinks. The PVP gems seem to do very well. Also use your knowledge of what gems each class uses and focus on those. Many times you can even vendor the green gems for a profit or at least to break even. Don't expect to get rich but to just recoup a good percentage of what you spent to level.
Once you reach 500 JC and can start crafting Pandaria gems things get exciting. You will get to do a research once a day and learn one new cut. When you do this you will also get an item, Facets of Research which will contain a couple of other cut gems of the same color, so you have gems you can sell right away without having to cut any. In Patch 5.2 a new patterns drops for JCs called Design: Serpent's Heart. You can make one of these a day and it will contain a random raw gem. This pattern is a pretty common drop and shouldn't take long for you to get.
You may wonder what to do with all your uncommon quality gems. There are 2 things you can do. First, make jewelery with them. On average 1 out of every 10 will proc to rare (blue) quality. Sell these on the AH. The ones that don't proc can go to your enchanter and become enchanting mats. This process is known as the shuffle. The other thing you can do is try to craft gems. Again you should get about 1 in 10 to proc as rare cuts (perfect cuts). These will have the same stats as standard rare gems but with a different name. Some people don't realize that these gems have the same stats as normal rare gems and will pay more for them. Put them up on the AH and see how it goes. The onese that did not proc you can just vendor for 85 silver each. Keep an eye on the AH for the uncommon gems as well. If you can buy them for less then 85 silver per gem that is free money! Do the above process and you will at least break even every time. As long as you get 1 perfect cut you made profit!
Start testing the waters of your market. Jewelcrafting is a highly competitive market on most realms and gems sell quick. This is good and bad. You may have to do some Auction House PVP but it can be very profitable. Good luck and have fun.