No matter how cautious you are, your cats are going to get sick, even if it's only every once in a while. If your cat gets a cold, perhaps the easiest thing to do is take it to a vet; she'll have the most effective, safe ways to treat the cold.
There are some things you can do at home though, which I'll talk about here.

Instructions

Sneezing, sometimes many times in a row, is a good sign of illness, as well as loss of appetite.

Running from your cat's eyes may be a clear liquid, which can dry on its face, leaving "tears" or dark streaks down its face.

Wheezing or other signs of odd breathing can point to respiratory problems as well.

Bring your cat to the vet for medication, though what they will probably give your cat is an antibiotic to prevent secondary infections or other diseases from taking hold.

Give prescribed medication to your pet as directed by the vet, for as long as you're told.

Ask your vet for examples of how to give your cat its medicine.

Clear away your cat's runny noses with a tissue, which can help with its discomfort.

Replace dry cat food with wet soft food, which has a pungent smell and will attract the cat more than dry food, according to PAWS.org. Your cat may not be able to smell dry cat food as well and may not notice when you put out food.

Put your cat in a warm, dry room while they're sick, PAWS.org advises. Do not let the pet outside.