Pennycollecter,
I have gone metal detecting several times. I have found some gold jewelry, and a lot of junk coinage. Nothing to really write home about. It is fun though, but I agree you should have a good back and strong legs to be successful.
You don't need a really expensive detector to get started, mine cost only a few hundred dollars. Interesting story actually, I ordered a Tesoro brand detector and the guy sent me a garrett brand detector. I was pretty upset, but it turns out the garrett works great and is a better detector that is easier to use. I just wish it wasn't yellow. I stick out like a sore thumb, being 6'4" 285 lbs while swinging a yellow stick around.
I don't like to be seen, not an easy feat with my stature.
If you do decide to get a detector, practice with it first. Don't just go out swinging that thing thinking you will find something your first time out. You could, but it most likely will not happen.
Practice swinging the coil over different metal objects to get a feel for the detector; pay attention to when it beeps as you swing the coil over. This will help you pinpoint better when you can't see the metal as it sits in the dirt.
Then I would suggest practicing in the sand. It is hard to destroy something like sand. One thing about detectorists, is that most people do not like us for several reasons. Most of these reasons are either ill informed, or the detectorist is a moron. You cannot just tear up someone's yard. You have to practice digging next. The grass you dig in should not look like there was ever a hole there. If it does, chances are you will upset someone.
There are certain tools that you will want to bring with you. The detector of course. A garden spade for digging small holes. A very long and skinny screwdriver to help aid in the pinpointing process--only as a last resort. Some sort of sifting tool if working in sand or other loose type dirt. A strong magnet can't hurt, even though you could get away without this one. Something to put your cool finds in.
Oh one more thing that I forgot to mention that belongs in the metal detecting 101. If you dig something, even if it's trash, you keep it. You can throw it out later, but never put it back.
Hope this helps,
Josh