I've just installed a new battery and starter in my 1988 Chevy van G20 and it won't start what can be the problem?

If the starter motor doesn't crank, read this link:

https://www.askmehelpdesk.com/cars-trucks/faq-how-troubleshoot-repair-maintain-hondas-selected-other-vehicles-46563-3.html#post235560

If the starter motor cranks, but the engine doesn't start, read this link:

https://www.askmehelpdesk.com/cars-trucks/faq-how-troubleshoot-repair-maintain-hondas-selected-other-vehicles-46563-11.html#post2123675