Ask your vet which meds to use; they know your local conditions, including any local resistance issues.
Generally we expect adult ewes to manage their worm burden unless it's a really horrid year. We worm twin-bearers at least on turnout/after lambing, then usually don't worm adults again unless there's a reason to.
Lambs need to be watched and action taken swiftly if there is a problem. We aim to get the first few batches away without any interventions at all, but this cannot be achieved every year. In particular, watch out for lambs not thriving, especially if their poo is dark/blackish. That could be coccidiosis (and it loves a cold spring); it's swift, nearly silent, and lethal.