The problem with you not showing any work is that we don't have any idea what you have to work with? Do you know that a rotation matrix must have determinant 1? And that the "length" of each column (thought of as a vector) must be 1?
If you know that, this problem is almost trivial. If you don't know it, do you know that you can find the matrix corresponding to a linear tranformation (in a given basis) by seeing what it does to the basis vectors? The coefficients of the result, as a linear combination of the basis vectors, form the columns of the matrices. With a rotation of , what does the unit vector (1, 0) rotate to? Its x and y components form the first column of the matrix. Do the same thing with (0, 1).