“开始就迈错了脚”，意思就是“一开头就很不顺利，出师不利”，例如：

His idea was good, but he got off on the wrong foot when he started doing it.

他的主意不错，可一开始就出师不利。

这个片语的反义词就是 get off / start off on the right foot，意思就是“有个良好的开端”，例如：

It's important to get off on the right foot in this new job.

开始一份新工作时有个良好的开端很重要。