mj leaves grow right off the main stalk. they grow in pairs and each new set of leaves grows opposite (or perpendicular) from the last set of leaves below it. each new set of leaves is called a node. so, a plant is said to be 4 or 5, etc. nodes high.
a cola is the flower cluster that forms at the end of a grow tip. that's the part you smoke. you can let your plant grow natural and many colas will form at the end of each grow tip, when flowering. or you can prune and train your plant to have only one cola, at the top of the main plant growtip (or meristem). many people claim that growing one main cola is the best. it will grow large and dank if you prune the plant so it concentrates on that one main cola.
hope this helps.