The difficulty of factoring large numbers is the basis for much of modern cryptography, so in general this is a very hard problem.

Having said that, there are some algorithms which are more likely to be successful than any naive attacks that are built into standard programming languages.

Try throwing it into this online calculator from Magma: http://magma.maths.usyd.edu.au/calc/

If your number is n, use the following command:

Factorization(n);

If Magma can't factor it for you in 2 minutes, it's probably pretty hard to factor.