Where should I use my own key?
I haven't used that library but it seems pretty straight forward. Take this part at the top:
// generate a random key and IV
var key = forge.random.getBytesSync(16);
And put your key in like this:
// generate a random key and IV
var key = neverGuessMahKeyIs1234;
Do the same for the iv
if you want.
Where can I choose 256 bit mode?
Ok, so first of all your dealing with symmetric encryption which has a key length of the desired size. Because it's symmetric, it's used on both the encrypting and decrypting ends, which is what the code that you posted seems to do. I say 'seems' because I'm trusting that the library's native functions are as you posted them. So, the code as you posted seems to use (as I showed above) 128 bits (16*8=128). If you want a random 256, then just use:
var key = forge.random.getBytesSync(32);
Or just make your own key that 256 bits long.