Basically, if the battery came with a connector on it, you plug that into the ESC's battery connection if it fits. Check the polarity first, obviously. For single battery setups, that holds true for 1, 2, 3, and 4 cell LiPos. If the battery did not come with a plug, you need to buy one that matches your ESC plug, if it came with a plug that is. A great number of ESCs and batteries don't have plugs attached. The reason is that there are a number of popular choices. And it's cheaper.