Biggie,
Being that it will be your first time, I would start with a 1/2 tab (mexican, 25mcg) and see how your body reacts. You should see a big difference in how fast you get hungry and burning fat at a much faster rate.
As to how soon to start? That depends in what type of shape you are in..... If your in great shape and keep getting leaner, you might only go up to 25mcg and leave it at that. Then again, if your metabolism is slow you might have to go higher. It really depends......
For me, if I go much higher than 50mcg's I start to burn muscle off just as fast as fat. So start low, and slowly (SLOWLY) work your way up.
Best of luck,
chris