Creating a plugin from html/php script

I’m trying to convert a php script to a WP plugin. I know very little php and html. I did look at the codex which is where I found the basic information to create the plugin and it works fine now as I have made the entire script a function which can then be called in WP. However, there is one thing I can’t figure out. The script takes input from the user in order to do some calculations and then change the original display of data. It does this using the html form construct.

If anyone could help me, I would appreciate it.