I would direct you to the tutorial, specifically Input No. 10.

To quote

There is no explicit "closed form" solution for a transcendental equation like this.

  You can find an approximate numerical solution using FindRoot, and giving a starting value for x.

Like so:

FindRoot[Sin[x]-x^2,{x,1}]
{x -> 0.876726}

I would direct you to the tutorial, specifically Input No. 10.

To quote

There is no explicit "closed form" solution for a transcendental equation like this. You can find an approximate numerical solution using FindRoot, and giving a starting value for x.

Like so:

FindRoot[Sin[x] - x^2, {x, 1}]

{x -> 0.876726}