The first system I emulated was the Chip8.. it's a bytecode from the 70's/80's kinda like java but much more primitive
it's very very easy compared to practically all other systems but is a nice little introduction to emulation. If you are able to emulate the Chip8 you can take on more complex machines. I suggest everyone who can code but have never programmed an emulator to give it a shot since it's such a kick seeing little games like pong and pacman running inside your program
here's all documentation you need:
http://www.goldroad.co.uk/ (click Emulation in the menu, then "A simple project to try" to the right)
It's a long tutorial about emulating the chip8, and at the very end is a link to documentation. there's some additional documentation and game roms (all of them 100% free and legal) that can be found through googling too. for example here's a handy opcode table:
http://members.aol.com/autismuk/chip8/chip8def.htm (nice table of all opcodes)
You can use any programming language to code a chip8 emulator, VB will be fine since almost no performance is required

(though I personally would recommend C/C++, C# or Java or similar since bitwise operations like shifts are more elegant)
Have fun!