Plot3D[f, {x, $x_{min}$, $x_{max}$}, {y, $y_{min}$, $y_{max}$}]See Drawing Option and Option Values for a list of Plot options.
Plot3D[x ^ 2 + 1 / y, {x, -1, 1}, {y, 1, 4}]
Plot3D[Sin[y + Sin[3 x]], {x, -2, 2}, {y, -2, 2}, PlotPoints->20]
Plot3D[x / (x ^ 2 + y ^ 2 + 1), {x, -2, 2}, {y, -2, 2}, Mesh->None]
Plot3D[Sin[x y] /(x y), {x, -3, 3}, {y, -3, 3}, Mesh->All]
Plot3D[Log[x + y^2], {x, -1, 1}, {y, -1, 1}]