Cat colds/flus are exceptionally rare. If it actually was a cold/flu, it would be a virus in which it just has to run its course. All you could do is make sure the kitty is eating and getting enough fluids. However, like I stated, a true cold/flu virus in cats is rare.
Now, URI (upper respiratory infections) is VERY common in cats and often is mistaken as a cold because it looks like it with runny nose, stuffed up, icky eyes, sneezing, ect. It is very contagious between cats (not at all to people) and is airborne. To treat it you will need to go to the vet for an antibiotic. Clavamox (clavamic acid w/ammoxicillian) is the most common treatment and comes in both pill or liquid. Also, if there are other cats in the home, they would need treatment too. Even if they don't have symptoms yet...as they may have it but not bad enough for symptoms so you just want to make sure incase they would get sick later and reinfect the home again.
If your kitty is not eating much or at all and not drinking much or at all please get to the vet soon. Also if the congestion is really bad that the kitty is weezing, get to the vet asap. I've actually since kittens die from this because it got so bad they just couldn't breathe.
Being that flu and colds are viral, there's nothing OTC that would help. However, you can get some powdered Lysine from any health store to help boost kitty's immune system. You start by giving 500mgs mixed in food and once kitty is better, drop to 250mgs as a maintenance dose
However, if your cat seems very ill and isn't eating, I would suggest you get kitty to the vet to find out exactly what's wrong since there's always the possibility of a secondary infection which WOULD need antibiotics. Also, your vet probably sells some type of Lysine that would come in either paste or gel form.