Quick answer
No quiero nada.
One no before the verb negates the sentence — and unlike English, a second negative word is not only allowed, it is required.
The basic no
No hablo francés.I do not speak French.
No, gracias.No, thank you.
Two negatives, one meaning
No hay nadie.There is nobody.
No tengo nada.I have nothing.
No voy nunca.I never go.
Nunca voy.I never go. — negative word first, then no "no".
ni … ni
No como ni carne ni pescado.I eat neither meat nor fish.
Memory trick
If the negative word comes after the verb, you need "no" in front. If it comes before the verb, you do not.
Typical mistakes
No quiero algo.
No quiero nada.
After a negation Spanish uses nada, not algo.
Tengo no dinero.
No tengo dinero.
The no goes in front of the verb, not after it.
Check yourself
___ conozco a ___.