想要变瘦，你可以使用以下几种英语表达方式：

I want to lose weight.

I aim to become thinner.

I aspire to have a slim figure.

I am determined to lose some weight.

I need to shed some pounds.

这些表达都可以有效地传达你想要变瘦的愿望。根据具体的语境和场合，你可以选择最适合的一种来表达。