Must be missing something

Must be missing something

Have created a simple form. Data is organized by item number 01 to 99. I want to open the form view, & when a user enters an existing number into the item number field, I want to display in the form and have available to change the data associated with this item. Submit button at bottom. Not rocket science, I know but we all have to start somewhere. I have chased around help and forums and can see all sorts of good stuff but cannot get a handle on where to start with this simple function. Any pointers would be great. Thankyou.