1998 Nissan Sentra wont turn over

Start with the battery. Make sure its fully charged and the cable connections are tight and not corroded. If it doesnt hold a charge, change the battery. Now try starting the engine. If the engine starts but dies again within a few minutes then your alternator is probably not charging. If, with the battery fully charged, the engine still does not start then it might be a starter problem. Turn the ignition and listen closely. If you hear a single click noise, then the solenoid or the starter might be bad. But thats not always the case, sometimes a bad solenoid doesnt make a noise at all. It could also be a bad ignition switch. I'm sure you can find tutorial's online that explain how to test each inividual component.