My bike is a Micargi RD-7 7speed road bike

So i don't really know anything about bikes so thats why i'm here. I need help with my bike with a part that broke. The part where the pedal screws into the crank arm?(i think thats what the part is called), well that part when i was riding came loose cuz i think my friend didnt assemble screw it in right and when i rode it on my mornin ride up a hill i tore the pedal out and then noticed that the inside metal screw in part was completely worn down so i cant even screw in my pedal now. Uhh so i dont know what part i need to replace. do i need a new crank? or crank arm or what? Im not looking for a great part just a replacement thats cheap. And can you explain if i need to to and how to measure for new part the size or anything. I know it has 52 teeth. Thanks for the help

I put up pics hope that helps.