Read write categories from form

Hi all,
I have low hopes of somebody actually answering that cause of the high level of spoon feeding involved 😉 But then again, I ll give it a shot.
I would like to make a simple form consisting of an input text field, a dropdown list of existing categories and a submit button. The logic would be:
Case 1: User inputs text to the input field and clicks submit. Then code checks if text equals name of existing category. If category name exists then nothing is done and code prompts user that this category exists. If category name does not exist then the entered text is being written in the database as a new category.
Case 2: User leaves input field empty and selects a category from dropdown list of existing categories. Then upon clicking submit, the code would store the category name selected from the dropdown in a variable.
Case 3: User clicks submit without entering any text or selecting any category from dropdown, then code does nothing and prompts for user to retry..

Any help, code or link, would be appreciated.

Harry