I'm a 16-year girl. I wear make-up when I go out. I wear short skirts and tight jeans. I want to know what should I do to repent?

Praise be to Allah, the Lord of the Worlds; and blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
Dear sister,
We ask Allah to guide us and you to the right path and to protect us from committing sins and from falling in Satan's traps. Know that repentance from all sins is obligatory instantly on all Muslims. Allah Says (interpretation of meaning): {And all of you beg Allâh to forgive you all, O believers, that you may be successful.} [24:31] Allah also Says (interpretation of meaning): {O you who believe! Turn to Allâh with sincere repentance! [66:8].
So, repentance is obligatory instantly, because the person does not know when death will surprise him and put an end to his ability to act. In fact, the old person, the young person, the sick person and the healthy one, all of them, are subject to die at anytime. None of them is closer to death than the other. And if the person dies without repenting from his sins he faces great danger.
True repentance combines the following conditions:
1. Regretting thoroughly having committed the sin.
2. Having firm resolution not to commit this sin again.
3. Stop doing this sin immediately.
4. Returning to people their rights or seeking their forgiveness if the sin is relative to people's rights, like stealing or backbiting …. etc.
Allah Says (interpretation of meaning): {Allâh loves those who turn unto Him in repentance and loves those who purify themselves (by taking a bath and cleaning and washing thoroughly their private parts, bodies, for their prayers, etc.).}[2:222]
Allah spreads His Hands during daytime as to give an opportunity for those who have committed sins during the previous night to repent from their sins. Allah also spreads His Hands during nighttime as to give an opportunity for those who have committed sins during daytime to repent from their sins.
This will continue until the sun rises from the west. (Doomsday) Allah forgives all sins no matter how bad and dangerous they are. Indeed Allah is the most Benevolent, The Oft-forgiving. Allah forgives all sins. Allah Says (interpretation of meaning): {Say: "O 'Ibâdî (My slaves) who have transgressed against themselves (by committing evil deeds and sins)! Despair not of the Mercy of Allâh, verily Allâh forgives all sins. Truly, He is Oft-Forgiving, Most Merciful. "And turn in repentance and in obedience with true Faith (Islâmic Monotheism) to your Lord and submit to Him, (in Islâm), before the torment comes upon you, then you will not be helped.}[39:53-54]
So, you should repent immediately. Do not hesitate. Know that death can come at anytime.
Know also that youth is too precious to be spoiled. It should be spent in the obedience of Allah, in what pleases Allah and in what benefits you in this life and the Hereafter.
Allah knows best.