Replaced starter and battery and now the car won't turn over. Why?

Well it was having trouble starting and I was tapping the starter with a hammer to turn over. I was doing that for about 3 weeks then the battery died on me so I replaced the battery first. Then the starter a week later

have you tested the small wire on the starter,while helper turns the key to the cranking position checking for signal/voltage from the ignition switch?

No. How would I do that?

volt meter set to DC VOLTS and A helper

OK and how many volts is it supposed to have to start it??

a fully charged battery is 12.6 volts

OK cool thanks. People are telling me that they think I got the cables switched on the starter when I put it on. I don't see how that's possible. I just put the new battery in 2 weeks ago so it should be good. I'm going to check it anyway though. Is there any other ideas you might have for me?? I'm hearing fuses, ignition Cole, crank shaft, and a few other things. I just don't really know what order to go in.