Its very simple.Just follow these steps.
1-add a text input object on your form.
2-click on ‘when page opens’ or ‘when form opens’ options on your right hand side.
3-select ‘Set Field’ behaviour.
4-In the properties select the object i.e. the text input ojbect that you created in first step.
5-then in value selection area,select ‘Special’ and then from the drop down list select ‘User-First Name’.
6-Preview your form and you;ll see your name in that text input object.