[1]Know the seven destructive sins the Prophet warned of:
1. Shirk (associating partners with Allah)
2. Magic (sihr)
3. Killing a soul Allah has forbidden except by right
4. Consuming riba (interest)
5. Consuming the wealth of orphans
6. Fleeing from the battlefield
7. Slandering chaste, believing women
[1]Avoid minor sins as well — persistence on minor sins makes them major.
[1]Repent immediately after any sin — Allah forgives all sins except shirk if unrepented. (Qur'an 4:48)