How can I improve this code for Project Euler 7?

Question

By listing the first six prime numbers: 2, 3, 5, 7, 11, and 13, we can see that the 6th prime is 13.

What is the 10 001st prime number?

My solution:

Answer 1

A number p is prime if it only divides by itself and 1. You are checking only for divison by 2, 3 and 5. This is not enough. Check for every number till p / 2, or better till sqrt(p).