RSA: How to calculate the private exponent?
If I'm understanding your question right, you want to obtain $d$ from given $n$ and $e$.

You'll have to factor $n=33=3*11$ and as $N=p*q$ you have obtained your $p=11$ and $q=3$. Now proceed as usual with calculating the inverse.

As pointed out correctly above, you can't easily generalize this approach to larger numbers as factoring $n$ will be infeasible. Now you see how to factor the number but for larger numbers even the best algorithms fail. Concerning your $e=3$ I'd consider using Coppersmith's attacks on low exponent RSA.