If you understand what retrolefty wrote (always use a resistor with a led), I can tell you what my basic advice is: Buy a second Arduino board, in case you do something wrong.
Don't worry about the USB plug. The USB plug won't wear out fast.
If you have a wire with 5V accidently touching a component or a sensor, they could be blown. So rewire a powered circuit only if you know what you are doing.