You must save or publish your form, then the user can fill it in.
If you are talking about preventing a user from clicking the Design button and changing your template, this can be fixed in the Form Options dialog box located in the Tools menu. It is the Enable Proptection check box on the General tab.

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Greg Collins [InfoPath MVP]
Please visit: http://www.InfoPathDev.com
How do I turn off design button in forms so that a user can fill in my forms?