You can find our geometry optimizer module integrated in many of our products. The usage depends on your goals, the input you have, your workflow etc. The easiest way to start is to use the Clean 3D function in MarvinSketch, which you can find under the Structure menu. This runs a structural optimization on the imported molecule using MM. The available forcefields are Dreiding and MMFF94, however, Clean 3D uses Dreiding.
We also have a Conformer Plugin in MarvinSketch, which can generate conformers for the input molecule.
In the terminal you can use our cxcalc command line tool, which have functions for these features.
I hope this helps you get started,