I assume that you want the entries of the matrix to be uniformly distributed between -1 and 1
One option is to use RandomReal
RandomReal[{-1, 1}, {10, 10}]
For a more general approach (where you may use later with other distributions, use RandomVariate
RandomVariate[UniformDistribution[{-1, 1}], {10, 10}]
HTH
yehuda