My question is in de header already.

I want to make a horizontal menu. When je click on one of the items, the sub-menu of that item has to appear in the side-bar.

Does anyone know how i can realise this? Whith a plugin or so?

Thanks
WebHeer

There, at least in the past, used to be plugins that would show sub-pages in the sidebar. Try a search on wordpress.org or Google.

The Plugin Widget Logic solved the problem for me.