You should create a new view of Datasheet type and set it as the default view. That's all.
To create a (datasheet) view from code you should use the Add method of the SPViewCollection as illustrated on the following page:http://msdn.microsoft.com/en-us/library/ms480493.aspx
As in the example on the page (see the VB example, as C# is not the same!), you should use the SPViewType.Grid enum value for the view type.
With an existing view (for example All Items that is a standard view type) you cannot do that, or at least it requires more tricks.