I want to learn 6502 asm but don't know where to start

Michal Kowalski's 6502 simulator is the best thing to experiment on and see how the instructions work. You don't have to worry about writing a full program, or initializing anything, which can be hard if you don't even understand the instructions right yet.

You just need an "org" directive before the code, so that it's assembler know where to place it in memory. When you want to try the program out, just hit the "assemble" button, and then the "debug" button. Then you can order the code to run all at once, step by step, etc (and you can use the debug windows to check out the status of memory, registers, etc).

I suggest you try out the simple things in the simulator, like adding and subtracting numbers numbers, shifting bits around, etc, to get acquainted with the instructions.

In my opinion, it's best that you learn 6502 ASM separated from NES stuff at first, because when you don't understand things it's easy to get the 2 mixed up. This simulator is pure 6502, it has nothing to do with the NES. Although you can use it to write NES programs and compile them into ROMs, the simulator will not simulate anything that's specific to the NES in debug mode, just the processor.