How to write a basic assembler in C?

Question

So im attempting to write a basic Assembler in C that will take in two files, one input and one output. The Input will contain the instruction and the output will contain the assembled code which can be seen using "od -x output.txt | head -5" for example. My question however is how can this be done for a branching instruction? Provided below is the add instruction which also produces a weird output i dont quite understand either, in hand assembly for "add R2 R4 R1" it should produce an output of "1241" but instead it produces an output of "4112". Why does it do this?

PR1.C

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

char *ltrim(char *s) {
    while (*s == ' ' || *s == '\t') s++;
    return s;
}

char getRegister(char *text) {
    if (*text == 'r' || *text=='R') text++;
    return atoi(text);
}

int assembleLine(char *text, unsigned char* bytes) {
    text = ltrim(text);
    char *keyWord = strtok(text," ");
    if (strcmp("add",keyWord) == 0) {
        bytes[0] = 0x10;
        bytes[0] |= getRegister(strtok(NULL," "));
        bytes[1] = getRegister(strtok(NULL," ")) << 4 | getRegister(strtok(NULL," "));
        return 2;
    }
}

int main(int argc, char **argv) {
    FILE *src = fopen(argv[1],"r");
    FILE *dst = fopen(argv[2],"w");
    while (!feof(src)) {
        unsigned char bytes[4];
        char line[1000];
        if (NULL != fgets(line, 1000, src)) {
            printf ("read: %s\n",line);
            int byteCount = assembleLine(line,bytes);
            fwrite(bytes,byteCount,1,dst);
        }
    }
    fclose(src);
    fclose(dst);
    return 0;
}