I want to create a custom decimal format field in sharepoint with structure #####.#####
. Example: If you enter 1.05
this field will return 00001.05000
I think you have 2 options there and indeed, the custom field is one of these options.
In order to create a SharePoint custom field, you can follow this link : https://msdn.microsoft.com/en-us/library/bb417414(v=office.12).aspx
Your needs is quite easy to develop. The most part is to learn how to manage a sharePoint custom field. In a custom field you'lle have two part :
The second option would be to calculate your desired format and write it in another column. You could program the logic to do this in a Workflow or in Event Receiver.
I think the first option is the most appropriate.