Could be a flaky ignition switch. Does it help to jiggle the key around?

Maybe a flaky clutch switch or neutral switch, depending on what transmission you have.

The starter motor or it's solenoid might be flaky. If you can make it start by wacking the starter with a broom handle, then this is it.

The starter is by far the biggest electrical load, so if all the other stuff works, it might still be a bad battery. But then it wouldn't help to wait a few seconds. You could try checking all your big battery cables...