On your menu bar, navigate to View|Toolbars|Forms to display the forms toolbar. Position your cursor where you'd like the form field to appear, then click the ab| icon on the forms toolbar. This will insert a text form field at the cursor position. You can double click (or alternatively, right click and select Properties) to view and/or change the form field properties.
Once you've inserted all the form fields you like, you can protect it via Tools|Protect Document|Forms. The password is optional.
Note...if you fill in form fields, then unprotect the form and subsequently protect it again, all form fields will be cleared or reset to their default entries, if any are specified.