If you want to tell your Chinese friends that you are on diet, you can say '我在减肥' -- wǒ zài jiǎn féi

我 (wǒ) means I
在 (zài) means am
减肥 (jiǎn féi) means on diet or loose weight
肥 (féi) means fat.

Diet is a daily routine for many Chinese girls even they are not over weight. Products like 减肥茶 (jiǎn féi chá yào, diet tea), 减肥药(jiǎn féi yào, diet pills) are very popular in China.