How do i attach a garden item on the outside aluminum siding?

What are you trying to attach? is it heavy? You could try the command hooks

They sell vinyl siding hooks that you can use. Here is a link that includes that idea plus some others:

What type of garden item. Command makes exterior strips that might work. Check out your big box display of command products and determine if that would work.

I would go with either a Command hook, or they also make exterior velcro that is pretty strong and sticks to anything.

Look in some of the catalogs which are on line - Carol Wright, Brylane, etc. I just saw the type of clip you are looking for but cannot remember which booklet they were in.