A girlie-girl can be one of two things.
Firstly, and most commonly, a girlie-girl is a female who exhibits an overabundance of stereotypically female traits.
For example: the girlie-girl may have a bedroom decorated entirely in shades of pink. She might wear frilly little dresses or otherwise excessively "cute" "feminine" attire. The girlie-girl will giggle constantly and blush easily. Any mention of sex/boys/kissing will elict mock shrieks and face-hiding from the girlie-girl. Note, however, that these last behaviors are often disingenous.
Note, also, that most everything about a girlie-girl is disingenuous.
Secondly, "girlie-girl" is a common slur used by bullies against those males they wish to demean.