How to create an api for web based application.?

Question

Hello,

I have to create an api for my web based application.? So far i have created and done api(ie dll) for the windows based application. Im new to websites (asp.net) so i need to know how to create api for the web based application which should be cross platform(ie can be accesible by java,php). The main reason for that is i can give the peoples who request to get my services can use my api and and meet their requierements.

I need help in this.

Sorry for the misuse of technical terms.

Any help would be appreciated.

Thanks and regards.

S. Vikneshwar.