Will not start engine clicks?

I started my car like usual went to put in drive but it wouldn't shift, then I noticed all the electrical was out air and heat radio out cigarette lighter out power windows out no blinkers just no power to anything electrical. Then I pushed this button by the gear shift held it down and then I was able to shift into drive as I was driving off the car revved high then low lost power then gained power so I went to AutoZone explained my issue they ran the test where they plugged it in under the dash told me it read P0500 speed sensor so I left on my way home about two miles away it started losing power really bad barely got it home if I stepped on the gas it would lower rpm and almost stalled so I barely gave it gas and it kept wanting to stall as soon as I got home put it in park it idled like normal and then it started sputtering like it was out of gas then stalled I tried to start it up but it just started making clicking noises engine won't turn over nothing I have no clue as to what I should do please help.