It was written in assembly language by a team lead by Margaret Hamilton (scientist) - see Her Code Got Humans on the Moon—And Invented Software Itself.

The code was written in 16 bit “words” which included one parity bit for simple error detection. Apollo Guidance Computer

The total memory available (no separate “disk space”) was 4 kilobytes of read-write memory and 72 kilobytes of read-only memory.

The programs in the permanent memory were entered in by hand by weaving wires through donut shaped magnetic cores. If the wire went through a core, it registered a 1 and if it bypassed the core, it registered as a 0.

How they entered the programs into the core memory

Here is a photograph of Margaret Hamilton, chief programmer, next to the stack of code in printed form:

Margaret Hamilton, standing next to listings of the Apollo Guidance Computer source code.

You can actually read the code itself, see The code that took America to the moon was just published to GitHub, and it's like a 1960s time capsule

The computer itself looked like this:

Astronauts used it by pressing the “verb” button, entering two digits, then the “noun” button then another two digits and pressing the enter button. There were other buttons on the display they could press as well.

And there’s an online emulator of the computer in javascript here: Online Apollo Guidance Computer Simulator

And see also The "Rope Mother" Margaret Hamilton