If there aren't any other sheepkeepers nearby who can give you a helping hand, you could take the boy lambs to the vet, and get the vet to help you. In either case, with someone experienced to show you, you'll soon get the hang of it.
If all else fails, then yes the vet can castrate using a knife, or some will use bloodless castrators. But that will cost a bit more, and the knife leaves an open wound and hence risk of flystrike. So it's best if you can get them ringed - but be aware, if you're not sure of your technique, it's all too easy to leave a bit of testicle in there. That's not only much more uncomfortable for the lamb at the time, but also leaves you with male rigs, who may father lambs when you weren't expecting it.