I need to know some really great dumbbell-based bicep exercises.

im a wrestler and i need to gain more muscle outside of lifting. what are some dumbbell workouts? (btw i only have ONE dumbell)

I currently do 12 reps on one arm without stopping and then doing the same on the other arm using just one dumbbell. Is that fine or is it better to have two dumbbells and alternate sides after each rep?

I want to grow up bigger bicep.I have two 20 pounds of dumbbell and I do 20 reps of 8 sets of dumbbell curls three days a week,am I over training please suggest me.