Hi,
There are quite a few weighted exercises you can do to hit your lats. Visit the
exrx lats section for a comprehensive list of exercises and example clips.
In my opinion the pull-up or weighted pull-up is by far the best single exercise for hitting your lats, and a bunch of other muscles. If you can't manage a pull-up then start with body weight jumping pull-ups and keep at these until you progress.
Second to this the standard lat pull down machine and some form of weighted row would be my second suggestion.
Hope this gives you some pointers.