¿Where to Start Development?

I'm new to SAP B1 development. The company where I work wants to develop their on addon for own use, but we just don't know where to start.

Some people say that we need a SDK license to develop or some other people say that we just need to start programming with VB.net using the samples of B1 installation without the SDK license, and now we are really confuse.

Maybe someone could help us giving some tips to what to do.

Thanks for your time!

regards