nesdev.org

You should probably try to find a GB Hello world type example that has the framework for building a GB ROM and then you're able to begin programming whatever it is you want from there. Otherwise you'll just have to take the time to figure out how the assembler and linker and all that works together. It probably isn't that hard to figure out though it may be a little intimidating at first.

I do recommend the GB USB SmartCard for testing whatever you make on a real GBC. It's cheap and works pretty well.

Or, use BGB. 99% of the times, it emulates things correctly. (Make sure you have the latest version if you already have a copy on your hard drive, however. There has been some substantial improvement in several areas over the last two years.)