This is assuming the lightning is shooting downwards.
If you want it realistic then remember that the first visible element of a lightning strike travels from the ground upwards a distance.
Anyway, one thing to consider is what counts as "realistic" in the context of a computer program - if it's an effect in a game you want then you really have to ask if procedurally generating and rendering the downwards path of a strike is going to be any more worthwhile than, say, just alpha'ing in a picture of one. What's the context? Because a player engrossed in a game unlikely to appreciate anything beyond a certain level of realism.