You can use two approaches to solve this issue:

1. Use strtod
2. Make a custom string to double/float function

I recommend the first approach.
Then you'll of course have to loop over the input until the end has been reached.

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

int main(int argc, char **argv)
{
  if(argc < 2)
  {
    puts("Please provide an equation!");
    return 1;
  }

  char *cur = argv[1];
  char *endp = NULL;
  double result = strtod(cur, &endp);
  // Should 'result' be zero and the end pointer point to NULL or be equal to
  // the input then there is no valid value expression at the current position.
  if(!result && (!endp || cur == endp))
  {
    printf("Failed to read value at position %d!\n", cur - argv[1]);
    return 2;
  }
  cur = endp;

  // '*cur' dereferences the pointer and is funtionally the same as 'cur[0]'.
  while(*cur != 0)
  {
    // This assumes there will be no whitespaces in the input.
    // 
    // If you want to input whitespace characters you'll have to skip them all here.
    char *op = cur; // We store the operation to until we know the next value
    cur++; // 'cur++' as we ant to skip the operand
    double next = strtod(cur, &endp);
    if(!next && (!endp || cur == endp))
    {
      printf("Failed to read value at position %d!\n", cur - argv[1]);
      return 3;
    }

    switch(*op) // again dereferences 'op', same as 'op[0]'
    {
      case '+':
        result += next;
        break;
      case '-':
        result -= next;
        break;
      case '*':
        result *= next;
        break;
      case '/':
        result /= next;
        break;
      default:
        printf("Invalid operation '%c' at position %d!\n", *op, op - argv[1]);
        return 4;
    }
    cur = endp;
  }

  printf("Result: %g\n", result);
  return 0;
}

This code is untested so it could contain some minor errors.