"このスカートはあたらしくないです。"

Translation: This skirt is not new.

[stanofmeel]

このスカート新しく無いです

[FarrelMahardhika]

You forgot the は between スカート and 新. Thanks, nevertheless.

[JohnPMChappell]

このスカートは新しくない。 For a more casual sentence.

[DijonDaddy]

It's pretty annoying when people put desu at the end of every sentence when its not needed in most cases.