First Tap touch event

Hi Im making a cocos2d game that has a sprite flying and falling, Im trying to have a first tap touch event for example when the user touches the screen for the first time it'll start the game animation and start the physics engine. Whats happening is that when the user starts the game the sprite falls down right away, can anyone give me a hand with this?

right now Im using something like this but Im not sure how to get the physics engine to wait until the user touches the screen for the first time.

    CCSprite *_pixie
    CCNode *_start;
    BOOL *_firstTap;
    CCPhysicsNode *_physicsNode;

    -(void)didLoadFromCCB{
         _physicsNode.collisionDelegate = self;
         _firstTap = True;
    }

    - (void)touchBegan:(UITouch *)touch withEvent:(UIEvent *)event {
         if(_firstTap == TRUE){
            _start.visible = FALSE;
           _firstTap = False;
         }

        //flying sounds & so on
         if (!_gameOver) {
           [[OALSimpleAudio sharedInstance] playEffect:MAGIC volume:0.4 pitch:1 pan:0 loop:NO];
           [_pixie.physicsBody applyImpulse:ccp(0, 420.f)];
           [_pixie.physicsBody applyAngularImpulse:11000.f];
           _sinceTouch = 0.f;
        }
    }

- (void)update:(CCTime)delta {

    if(_firstTap == FALSE){
        float yVelocity = clampf(_pixie.physicsBody.velocity.y, -1 * MAXFLOAT, 200.f);
       if ((_sinceTouch > .5f)) {
        [_pixie.physicsBody applyAngularImpulse:-40000.f*delta];    
       }
    }
}

Change

BOOL *_firstTap;

to

BOOL _firstTap;  //No asterisk

And also make sure that you set _firsttap = YES in viewDidLoad functions

- (void)viewDidLoad
{
    [super viewDidLoad];

    _firstTap = YES;
}