Super sets are the way forward
But I wouldn't advise soley training your bi's, atleast do some triceps too to balance it out.
Preacher / Peak curls
Zottman / Hammer curls
Followed by some negative curls of your choosing. Eg Conc curls.
You can google all these if you don't know how to do them, but they're all doable with dumbbells.
Switch it up from session to session. Leave a couple days rest in between training. Eat a high protein (and carb) diet and you will see results
EDIT: With regards to weight, it'll take you time to learn what weight is perfect for you, then from here you can continually add weight to further gain muscle.
8-12 reps range is perfect for muscle building. Lower if you want to focus on strength, higher if you're more looking to cut up.
What I mean by 8-12; Add some weight on to your dumbbells, start doing Preacher curls for example, get to the 9th rep and that physically not be able to do another single rep. This is the perfect weight for you. If you CAN do another rep, but you choose not too. Then you need to add more weight and TRY harder
Good luck dude !