This list is intended for lipo newbies, who want to run a typical bike kit controller on 12s 10 ah lipo. So that's your generic 36-48v controller.
First off, you can buy whatever c rate lipo you like. The better the battery you get the better, so go as high c rate as you can afford. If you are on a budget, the 20c ones will do fine. More impotantly, buy one that's in stock. Don't even click buy on out of stock batteries. Turnigy or Zippy, whatever is in stock, and in your budget.
So you want 44v 10 ah. Buy 4 5000 mha (5 ah) 6s bricks.
You want to monitor voltage while you ride. This is the cheapest way. Buy two. Low voltage warning light that goes off at 3.75v. At 3.5v it beeps at ya. Some soldering required. http://www.hobbyking.com/hobbyking/stor ... oduct=7227
You might want to monitor with these, for cell level monitoring. If so, buy two. Handy to have one, in any case. No soldering. http://www.hobbyking.com/hobbyking/stor ... duct=10952
At the moment the beepers are out of stock, so buy something similar that is in stock. Or hunt them down somewhere other than HK.
Now you want to connect it up. What connector you use is highly personal, and may also depend on whether you want a permanently paralelled pack or not.
So get out your thinking cap, and figure out what you want. What connector, connect permanent, or connect temporary. You want to end up with two packs of lipo, consisting of two packs connected paralell. You can cut the wires, and solder them together so two packs have one of the bullet connectors of each color instead of 2 pairs. Or you can make up a paralell harness that any lipo brick can be connected to, using the same bullet connectors. Or you can cut off all the connectors, and install your favorite type of connector. Maybe andersons, Maybe something else. If permanently paralelling the batteries, then you would want to connect all the balance wires similarly. Most just use some solder and heat shrink. Or you could make a jumper with a male jst plug on each end. Many many variations.
Next you want to series connect the packs. With andersons you would need no special harness, just connect red to black. With other types of connector that are polarized, you'd have to make up a special adapter.
So you want some more connectors and wire for that method. You may need additional wire to reach your controller location, and that could be built into a series connect harness. You may want to attach your lv warning devices to the harness, or you may want to plug them into your balance plugs. In any case, you may need wire and more connectors of some kind to reach the location you plan to mount the device on the bike.
No way can I tell you what to buy exactly, since there are a lot of connector types, and strategies for permanent or temporary paralell connections. There are threads that show what I have done, in the lipo noob links thread.
Now to charging. Again, a zillion ways to do it, but I recomend two chargers. That way, when you disconnect your pack to charge, you can charge both packs at the same time, with a simple plug it in operation. One S350 Meanwell power supply is a great start. Buy the right voltage for your charger. Many use 12-18v power, others use 24v. The meanwell is a 350 watt supply, so now select two chargers of up to 150 watts each. That way the power supply puts out 350 watts and your chargers are taking 300. Make sure whatever connector you put on the packs, you buy some more, to put on the charger.