I'm guessing that these are gimmers you have - about 18 month oldand ready to tup, to lamb for the first time at 2 years old or thereabouts.
How long have you had them? Did they all come from the same flock? Had your ground had sheep on it in the recent past? Did you do any worming when they arrived ie quarantined?
Depending on the answers to the above, you may never need to worm them. Otherwise, as Shep said, talk to your vet about a faecal worm egg count.
Adult sheep generally don't need wormed - they build up a resistence. Young lambs, like those you'll have next spring, don't have that resistance so worming might be needed once they start eating grass at about four weeks old. We used to worm the ewes at lambingnot fofr their benefit but to stop them shedding worm eggs on to the pasture, that would, in turn, infect the young lambs.
You do need to talk to your vet about fluke though. The vast majority offarms now have it; it's carried by deer, rabbits and just about every other herbivore and sheep don't build up a resistance.
Run a mile from combined fluke / wormers. Chances are you'll be treating for something unnecessarily. If you vet is anything like mine, you'll be able to buy 6 doses of the right wormer from them. More expensive per dose but saves on waste and waste disposal.
Make sure you give the right dose for the sheep weight and make sure they swallow it all. SCOPS has information on dosing too.