(define macros '()) ; an a list of macro definitions.

(define (define-macro name expander) (set! macros (cons (cons name expander) macros)))
(define (find-macro name) (let ((a (assoc name macros)))
                             (if a (cdr a) #f)))

And add to your eval:

(define (eval form ...)
   (cond
      ...
     ((find-macro (car form)) (eval ((find-macro (car form)) form) ...))
     ...))

So now you can start to write macros such as:

(define-macro 'if (lambda (form)
                      (let ((test (second form))
                            (then (third form))
                            (else (fourth form)))
                        `(cond (,test ,then) (else ,else)))))

People here will cry, because there's no hygine, etc, that's why I
qualified it of quick and dirty.  But it can help.

Now, of course, for define expanding to define it might be difficult.
You'd need a primitive define, let's name it %define (change your
eval), and then you can write:

(define-macro define (lambda (form)
                       (if (list? (second form))
                           ; (define (f . args) . body)
                           `(%define ,(caadr form) (lambda ,(cdadr form) . ,(cddr form)))
                           `(%define . ,(cdr form)))))

and be happy.