Note: At other times of the year, there is a fourth step of repentance according to the Rambam. Before taking these other steps below, you must recognize what you're doing is a sin and stop it. Since Yom Kippur is covering the last year's worth of sins, I'm presuming that the sin is over and completed. But if you have a sin that repeats itself, stop it first.
Confession: You need to say your sins out loud, whether that is only to Hashem's ears or to the human ear. Normally, this happens during the Ten Days of Repentance and Yom Kippur with a fixed text that covers just about everything you could have done. However, you may add to the list whatever you need to.
If your sin was against a person, you must confess your sin to that person and seek forgiveness. But if you honestly seek forgiveness and offer restitution three times but the other person denies it, you don't have to ask any more. At that point, the other person bears the sin of holding a grudge.
Regret: You should feel bad about your sin. Don't rationalize it away. Think about how it has damaged you and your relationships with Hashem and other people.
Teshuvah: Turn away from the sin by resolving not to do it again in the future. If applicable, create a plan for avoiding that situation in the future or how to better handle it.
As preparation for Yom Kippur, review the Al Chet prayer and spend some time thinking about the last year (or years) of your life. Chabad has a copy of the Al Chet here.
Here is a final thought for you on "chets." Chet means sin in Hebrew, but that's not its literal meaning. It originally meant to "miss the mark" in archery. I think this is important to remember when we try to rationalize that one sin or another wasn't really that serious, no one was hurt, and maybe it's not such a bad thing in the first place. In archery, you hit the mark or you don't. But at the same time, you can always be working closer and closer to the mark. So maybe you have the same list of sins as last year. Consider where you've moved closer to the mark and maybe where you've moved even further away.