As someone who is legitimately a Christian, you are also a new creature. The things of the world you once enjoyed should no longer be pleasurable to you. If you drank liquor, the Spirit of God should convict you of that. If you were one who spent time in dance clubs, and you are saved, the Spirit should have shown you that modern dancing is a sin! Likewise, when you were saved, the Spirit of God should show you that immodest apparel is a sin also. As a new creature, you have the individual soul liberty to do what you want to do, but what you want to do should reflect becoming a new creature through salvation.
So what should a modest lady avoid? Over the years, styles of dress have changed. Sadly, the styles of dress have become more revealing to the point where many women, whose character is far from harlotry, are dressed in ways which could easily cause a man to stumble into sin. Hollywood, the popular music culture and the fashion industry all encourage you to show off your body, but a Christian lady should know that her body is not her own.
1Cor 6:19-20 What? know ye not that your body is the temple of the Holy Ghost which is in you, which ye have of God, and ye are not your own? For ye are bought with a price: therefore glorify God in your body, and in your spirit, which are God’s.
As a Christian, you have been purchased already. Paul referred to himself as a servant. Literally, a bond-slave belonging to Christ. Therefore, since we are not our own, we belong to Christ and we should do the will of Christ. For a lady, dressing modestly is the will of Christ.