1996 Chevy Astro New battery not starting van

I just recently replaced my battery with a brand new Optima red top and the engine will not turn over as if it is dead still. It's not the battery as I confirmed it's good at auto parts store before bringing home (bad battery sold previously at another store). I can jump start and it will run. Is this an alternator or starter problem? Anything else I coud check before removing alterator as it will be hard to remove and check.