Almost anything carbon based will work well for fluxing. Some people use paraffin, others use pieces of candles, beeswax, old cast and lubed bullets, bullet lube, sawdust, or any number of other things to flux their alloys. The main purpose of fluxing is to remove impurities and alloy the good metals together. It doesn't take a lot, but could (and often should) be repeated to get your alloy clean. The amount would depend on the quantity of lead you are melting and how dirty it is.
If you are going to be casting for black powder, the pure lead is great. If you plan on casting for handguns, mix it 50/50 with wheelweights (clip-on type) to harden it up a little and then try to stick below about 1200fps or so. If you are casting for rifles, you may want to mix it with some linotype or something to get the hardness up a bit higher to allow for higher velocities or use gas checks on the bullets.
Good luck, be careful, and PAY ATTENTION to what you are doing. There are many great sites around for additional information on bullet casting. I'd recommend reading up on it and learning as much as you can so you don't have to learn the same hard lessons others have. I don't want to list the sites here, but if you want some help finding them I'd be happy to PM you a few of my favorites.
Happy Casting!