Start Gaussview from the Start/Programs menu (Windows) or the OS X Dock (Apple). If you use a Mac, and the latter does not succeed, try instead to open a terminal window and from this, type gv. [There seems to be a subtle problem with the internal Unix environment which causes this, it will hopefully be fixed soon].
Gaussview is generally used to initialise Gaussian Molecular Orbital calculations using pre-built structures derived from either Chem3D or CAChe and which have been already saved in MDL Molfile format (with a .mol filename extension). Ignore the Fragment window that appears when you start the program and the empty molecule window, and proceed to File/open to import a pre-built structure. You can if you wish use the GaussView editor to construct a molecule from scratch (use the built in help pages for details). Use Edit/Clean to produce a meaningful 3D initial structure; this is not quite as reliable as a proper Molecular Mechanics calculation, but for many cases it may be adequate.
Job Type: Optimization (optionally select Optimization+frequencies if you dont mind waiting a bit longer); Method: Ground State, Hartree-Fock, 6-31G Basis set; Link 0: %chk=yourfilename.chk, %mem=256MB; Guess: Save orbitals to checkpoint file. The checkpoint entry is particulary important, since you will use this file to generate MO diagrams etc. [You can try other options of course if you wish]. Press submit.
