I want to lose weight.
Stress 'PER-de-re' and 'PE-so'. Clear, even syllables.
State this as your fitness goal when speaking to a trainer during an initial consultation or when requesting a workout plan tailored to fat loss.
'Perdere peso' is the standard phrase for losing weight. 'Voglio' = I want (from 'volere'). An alternative is 'dimagrire' (to slim down/lose weight as a verb): 'Voglio dimagrire' is equally common and natural.
Voglio dimagrire.
I want to slim down.
More colloquial alternative.
Voglio bruciare i grassi.
I want to burn fat.
More technical, fitness-oriented.
Voglio tonificare senza perdere peso.
I want to tone up without losing weight.
For those who want muscle definition.
Italians typically measure weight in kilograms ('chili'). It is culturally normal to discuss weight openly with a trainer. Italian gyms often provide body composition analysis ('analisi della composizione corporea') using bioimpedance scales, which is included in many memberships.