The only 'healthy' diet pill is actually a supplement --- green tea extract. That is very good for your body --- lots of anti-oxidents --- and also ups your metabolism. Or you could drink lots of green tea -- it has a bit of caffeine, so that stokes your metabolism too.
But the best diet, of course, is lots of healthy, whole foods & fresh water & exercise. Then your body will be the best size for you.
Good luck!