I use Drontal or Milbemax for worms.
Advantage for fleas.
Both can be bought on the internet, or from your vet. Off the shelf stuff - at best it doesn't work. Worse case scenario it poisons. Bob Martin for example. Flea collars can also poison.
If you're going to use a collar, ensure it's one of the snap off, fast release ones. Not the elasticated type, which can cause horrific injuries.
Don't let them out until they've been neutered.