The question "What is some?" is incredibly broad so I will focus only on the sentences you provided. If the noun is not countable, you'd use "etwas" or "was" just as Robert suggested in his answer.
Ich will auch was.
If the noun is in its essence countable, you'd use "ein paar". "Etwas" wouldn't work then.
Ich will auch ein paar.
Now, a pizza is countable but the whole of one pizza is not. You cannot have one of a pizza. You can only have one slice. So in this case you'd use "etwas". But for nachos for instance "ein paar" is better,because you want several items.
If you add "of that" to your sentence, a second notion comes in and "ein bisschen" might also be the best translation.
Ich will auch ein bisschen davon.
But that is subjective I suppose.
And just to make sure... the irgend-words will NOT work in this context at all.