With yesterday's explosion of interest in sin as defined by the Roman Catholic Church, and consequent misinterpretation by the media, I thought I'd point to where you can find out more what constitutes sin.Frank Ritchie of Servant's Thoughts has written a thoughtful piece on what sin is.
The Roman Catholic Catechism goes into more explicit detail on what would be considered sinful. The list is far larger than the Seven Deadly Sins and the "new" sins being eagerly talked about.
If you still need help after reading all that, I'd recommend The Sinner's Guide by the Venerable Louis of Granada, OP (1504-1588). The online book goes through all the reasons why not to sin and the rewards for trying to live a sinless life. As well as remedies for each of the Seven Deadly Sins.
Ultimately sin leads to separation from God. It's not a good thing and leads to deep unhappiness, followed by possible eternal damnation. As much as it would be nice to think that it all ends when we die, unfortunately (or fortunately) we are immortal beings that will live forever either with or without God. Guess which option is the better one.