You would need two layouts. A View Layout and a Edit Layout. Both of these layout would be the same except the View Layout would contain Merge Fields and then The Edit Layout would contain regular fields and the buttons would be different. You would go to the view layout and then when you wanted to edit you would click an edit button on the view layout and then on the edit layout you would have a save and cancel button. The save button would go back to the viewing layout and the cancel would revert the data and then go back to the viewing layout.
Instead of Merge Fields, you might just use Duplicate Layout to duplicate your data entry layout, then select all the fields on the layout and use the Behavior settings to deny access ot the field. That's a lot less work and also produces a layout where you can permit the user to perform finds by entering criteria even though they can't edit the data.
There are also ways to selectively deny access to records for data editng via Manage Security. See "Editing record access privileges" in FileMaker Help and check out this particular sub section: "Entering a formula for limiting access on a record-by-record basis" for a description of how to set this up.