Can someone point me into the direction I need for this problem?

Consider the following nonlinear differential equation, which models the unforced, undamped
motion of a “hard” spring that does not obey Hooke’s Law. (Here x denotes the position of
a block attached to the spring, and the primes denote derivatives with respect to time t.)

x''+x+.01x^3 = 0

I have to put this into an m file to generate a numerical solution of this system over the interval of

0  t  6 for the following two sets of initial conditions.
i. x(0) = 1, x0(0) = 0
ii. x(0) = 2, x0(0) = 0