Why is your 2000 s-10 starting only when you hold accelerator to the floor?

Usually this indicates a super-rich fuel condition. If it makes a lot of black smoke when it starts that would also be a good indicator. Common causes of a super-rich condition at startup would be low coolant level; leaking fuel injector; leaking fuel pressure regulator; faulty or disconnected MAP MAF or ECT sensor. Any super-rich fuel mixture problem should be corrected ASAP as it can cause oil to become diluted with fuel and lead to major engine damage.

You could also have a problem of worn spark plugs, oil or coolant fouled spark plugs, or a minor timing problem. Do you also have a running problem such as misfire while accelerating or check engine light on or flashing? If ignored this can lead to damage of catalytic converters.

If you don't really have to hold the accelerator to the floor, but just press it slightly, consider a stuck IAC. If IAC is found to be At Fault, clean the throttle body when replacing IAC.