You can use the Loaded event to start your storyboard
See MSDN for an example:
http://msdn.microsoft.com/en-us/library/bb979882%28VS.95%29.aspx
Picked the example from MSDN:
<Canvas
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
<Rectangle
x:Name="MyAnimatedRectangle"
Width="100"
Height="100"
Fill="Blue">
<Rectangle.Triggers>
<!-- Animates the rectangle's opacity.
This is the important part, the EventTrigger which will start our animation -->
<EventTrigger RoutedEvent="Rectangle.Loaded">
<BeginStoryboard>
<Storyboard>
<DoubleAnimation
Storyboard.TargetName="MyAnimatedRectangle"
Storyboard.TargetProperty="Opacity"
From="1.0" To="0.0" Duration="0:0:5" AutoReverse="True" RepeatBehavior="Forever" />
</Storyboard>
</BeginStoryboard>
</EventTrigger>
</Rectangle.Triggers>
</Rectangle>
</Canvas>
The object Rectangle has properties. In the Triggers property we defined an EventTrigger which will fire when this event will occur. We choose the Rectangle.Loaded event, which means it will fire when loaded ;).
We add a BeginStoryboard property to begin our storyboard, and add a Storyboard. This animation will use a DoubleAnimation on the Opacity property, which means that in a duration of 5 seconds, the opacity will gradually fade to zero, and back (AutoReverse property) and it will do this Forever (the RepeatBehaviour property).