How much damage does a lightning bolt deal?
5 damage or 2.5 hearts, and possibly igniting you or the ground you are on. The wiki is not so clear on the damage a player takes, but the other answers have proven this. You may also be struck multiple times.
Most entities struck by lightning are dealt 5 half hearts damage, and are set on fire which may cause additional damage. - Mincraft Wiki
When lightning strikes, all entities within a 6×6×12 region horizontally centered on the northwest corner of the target block with the bottom edge 3 below the target block are struck by lightning. Multiple passes are made over this region, so items dropped during an earlier pass may be destroyed during a subsequent pass; damage immunity usually prevents struck mobs from taking more than 5 half hearts damage.
Can it kill me with a direct hit?
If the player is killed by a lightning strike, the death message will read [player] was struck by lightning. - Mincraft Wiki