I am currently using pay pal for my payment processing

however i am looking at how to use the api or instant payment notification can anyone help me?

I've been using it for years. I wrote a script once using the API and never touched it again as it seemed robust.

Which language is the API for?

Just to note and I'm not sure it matters, but I do have a business account with PayPal and I'm able to use their Sandbox to emulate real time purchases. Their Sandbox is really a life saver for me. I'm so glad they have it available.

A couple of places to look for info (aside from PayPal itself of course):

ospaypal.com - has information on how to use the sdk, or avoid using it

paypaltech.com - api sourcebook is especially helpful (has example SOAP requests and responses, etc.)