The fact that one testicle is larger than the other is perfectly normal. However, the fact that you can feel a small, painless lump in your scrotum is troubling.

Get yourself to a doctor ASAP. It's probably harmless, but you've got an early warning sign that definitely needs to be checked out by a pro. You're in the right age range to be at risk for this kind of thing (younger men are at higher risk than older men). It's probably nothing, but if it is something serious, you could save your own life by seeking medical attention now.