how can i add attachment field

Do you mean like, adding an upload field for your form fillers to attach(upload) a file? All you need to do is to add a File Upload field from the Basic Fields tab. 

Here are some guides that you will find useful: 

https://www.jotform.com/help/33-Changing-the-upload-size-limit 

https://www.jotform.com/help/29-Which-file-types-are-allowed-for-the-upload-field 

Let us know if you need more help, we will be glad to assist you.