- Repentance is stopping the bad things and doing more good
- You have to do it because otherwise you are not admitting that God is good and you are not good
A little longer:
Before becoming a Christian, everyone does what they want to do. For many that is even trying to be good according to what they, themselves, think is good. We are influenced in our understanding of "good" by many different things and people. At the end of the day, if we aren't looking to God to guide our actions, we are not good.
Repentance means deciding that God is much smarter and wiser than yourself, then finding out what He has to say about life, and acting and believing in your life the way God says you should. Where there is desire to be godly, God will give the ability to change, and during your life you will overcome your old ways and gradually be transformed into something new!
If you've properly understood what repentance is then it easily follows that you have to do it to be saved. If you didn't change your ways you would be admitting (by your actions) that you don't really believe God is worth listening to. You would be putting your own view of "good" above God's view.
Something to notice is that while repentance leads to doing good things, it is primarily a change of attitude. Just deciding to be good, and then doing good things (often called "good works") will not save you. You have to believe that God is the measure of what is good - this is a part of the essential saving faith.