ARe you using FileMaker GO on an ipad or iphone or FileMaker Pro on a computer? This question has been posted in the Filemaker GO section of the forum...
Since you are using a mouse, you must be using FileMaker Pro. Enter layout mode on this layout and select Set Tab Order... from the layouts menu.
Now set up the desired tab order for your fields.
Also, select each field one at a time and check the Go to next object using settings found in behavior section of the Inspector's data tab. This is were you can specify which combination of three keys can be used to move you from field to field according to the specified tab order.