Little bugs and how to get rid of them

I have tiny little bugs on the leaves of my plants. The adults look like tiny black flies. What is an effective way to get rid of them?  I have a small garden that I was keeping covered in a "green-house" sort of way with heavy gage plastic. Someone said not to do that but I had to cover it during all these rain storms or it would have been flooded.  But in the meantime, what can I use to rid my plants of these little buggers?