it depends on the number of bonds and number of free electron pairs.
So with H2O the Oxygen atom has two bonds and two electron pairs, therefore it is sp3.
the carbon in C02 has two bonds (countdoubles and triples as a single) so it is sp2
If I recall correctly only Sulfur and Phosphorous use the sp3d and sp3d2 orbitals (I forget which uses which)


But the best part is, when you get into higher level college courses you'll learn that hybridization is only a model, a flawed model biggrin: