Bikram Yoga works like a hot damn, (no pun intended) but it takes time. So take it easy, stand still, breath, breath slowly, calmly, just try the poses a little bit. If you feel dizzy, slowly kneel down and calm yourself, then rejoin.
It takes at LEAST 10 classes for your body to get used to the heat, and it can take YEARS to master the poses. But, fear not, any little bit that you can do will do you worlds of good.
Keep trying, hang in there and good luck! Report back on how it goes.