If you think you can get away without this, just stop. I did, and it only brought me grief and wasted time. I decided to get some and I found this post on Reddit from a user named Mr.MuffinMan91 helpful in the endeavour.
Let me give you a quick overview of flux. Flux is a semi-aggresive acidic mixture that has two purposes. The first is to remove the oxide layer on your board so that the solder can bond. The second (and sometimes under-appreciated) purpose is to allow the solder to flow freely. If you have ever tried to reheat a solder joint without using extra flux you know how important flux is. When the solder doesn't flow freely you are very likely to end up with solder bridges on small chips that are hard to detect. The main quality that defines a flux is how aggressive it is. A more aggressive flux will strip oxides better and allow solder to flow better than a less aggressive flux.
There are three types of flux you can buy: No-Clean, Rosin, and Water Soluable.
No Clean: This is the least aggressive type of flux. It doesn't strip oxides very well and it doesn't allow solder to flow very well. The trade-off is that the residue won't corrode your board. This means you don't have to clean the board (hence the name). Manufacturers love this stuff, but I don't like it for hobbyist use because it just doesn't really do much.
Rosin: This is the middle ground. Rosin flux performs well in both oxide-stripping and solder-flowing. The residue won't really corrode your board if left on (it will but very slowly), but it is recommended that you remove it. Removing rosin flux requires Isopropyl alochol.
Water soluable: This flux is very aggressive. It strips oxides very quickly and has excellent solder-flowing properties. I like to use this flux because it is very good at removing or preventing solder bridges. Residue from this type of flux will corrode your board fairly quickly if not removed. By very quickly, I mean in a matter of months usually. To remove the residue you need to wash your board in 130F (55C) water (that is hot enough to give you bad burns).
I use water-soluable flux on most of my projects, but I think that most people would rather use Rosin. I would recommend avoiding no-clean. Also, you typically don't want to mix flux types, so if your wire solder uses rosin flux then you should use a rosin flux pen too.
You will also find flux in either liquid or paste form. Paste is useful if you are soldering chips using wire solder and not solder paste. The tackyness helps keep the chip from sliding around. Liquid flux is good in most cases, and is easier to apply since it spreads across multiple pins.
I wouldn't worry about flux causing cancer. Just make sure to use it in a well-ventialated area and wash your hands after using it. The main ingredient in flux is usually isopropyl alcohol or some other solvent.
Personally, I prefer kester flux for my projects. My favorite rosin flux is type 186, and my favorite water soluable is type 2235. Both are sold in flux pens or by the gallon (if you want enough to last forever).
He mentions Kester Flux which can be expensive, but I manage to find a Kester rosin flux pen for 22$ CAD. I imagine I will outgrow the pen and need more soon, but it's a start.