There is a propery called "TabIndex" that you should use.
http://msdn.microsoft.com/en-us/library/system.windows.forms.control.tabindex.aspx[
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You can also change it directly in the forms editor.
You can just set the TabIndex of textBox1 to 1 and so on.
If you want to do it in code then you could use:
textBox1.TabIndex = 1;
textBox2.TabIndex = 2;
textBox3.TabIndex = 3;
...