Creating a bot as a web app

I am planning to create a membership website where people can pay the monthly fee to use the software. What I want to do is people will input some values and I will run a program in the background. Program will be running with their input even if they close the website. But the program that is running has to be unique for every user, the program will run with different parameters for every user. I will need instances of the program running on a server. Which tools, programming languages do I need to make this? I am not experienced with web apps but I am really good with HTML/CSS. Can you guys help me get started. Thank you. Instead of making a desktop app and letting users use it with login, I want them to input the values online and start the bot on the server.

Thank you in advance.