Count up the total number of "items" around the central atom. "Item" refers to either a covalent bond and/or a pair of nonbonding electrons.
If the total is two, the answer is sp (b/c it's an s and p orbital that mix).
If the total is three, the answer is sp2 (b/c it's an s + p + p)
If the total is four, then answer is sp3 (b/c it's an s+p+p+p)
If the total is five, the answer is sp3d (b/c it's an s+p+p+p+d)
If the total is six, the answer is sp3d2 (b/c it's an s+p+p+p+d+d).