how do I bind a UserID textBox to a Sql server database table

Are you using MVVM . if so then in the view model create the property with name UserId

public int32 UserId{ get{...} set {...}}

then in viewmodel call methods like wcf/webservice or sql call and get the data and assign it to the property UserId you created.

Then in the XAML page :

<textbox text="{Binding UserId}" />

And I think you will get what you want.