This project is fairly complicated as a first Arduino project.
I would start out with a simple proof of concept (and skills) prototype.
Make a small interface with say 8 sitches / leds. Figure out how to hook up and program everything, make it work so the LED's have the brigthness you want, test it. You can always recycle the components for your real thing.
When everything is working to your satisfaction, scale it up. I would go for some kind of modular approach.
This way you dont run the risk of ending up with a lot of LED's that are the wrong kind, a lot of switches that don't work, a lot of current limiting resistors for the LED's that are the wrong value etc. etc.
Things you need to know about before starting:
- switch debouncing, EXTREAMLY important for a musical project.
- pull up / down resistors for switches, a must to understand.
- current limiting resistors for the LED's.
- total current consumption for your project. more than 100 LED's will require a serious power supply if many of them are on at the same time.