If you already know some high-level programming languages, you can learn a little Z80 assembly and that will get you started with the Game Boy. I recommend downloading BGB emulator due to its excellent debugger. You didn't actually mention what you wanted to do with your hacking: it's hard to recommend anything when you don't say what you want to do.
Hacking is a broad topic, but most of us got started by messing with the ROMs and seeing what happens.
The simplest way to start, I suppose, is to hack some graphics. Load Super Mario Land in Tile Molester and turn Mario into Daisy.
Or you could make an infinite lives hack by using the RAM search function then use breakpoints to pinpoint the exact instruction where your life is deducted, and change that instruction. The important thing is to just do it. ROM hacking ain't like flying a plane: you can mess up as much as you like before you get it right.