Went to get a smog check and i failed because apparently my carbon levels were to high. Does anyone know how i can fix this?

They tried to charge me an extra $70 on top of what i had already paid to have them tell me what's wrong with it "diagnostic". If it matters, its a honda civic 90

Guess what? That $70 diagnostic was probably a bargain compared to what it will cost you to diagnose the problem now. We can't tell you how to fix it without knowing the problem. You can't learn the problem without diagnosing it. Since you obviously lack the skills and knowledge and tools, then you'll have to pay a shop to diagnose it for you, and it's likely to cost more than $70 to find out what needs fixing. Sorry.

When the fuel tank is almost empty, try some fuel system cleaner (Techron, Gumout, or Seafoam) and then drive it on the freeway for about 40 minutes.

Get the plugs, wires, air filter and fuel filter replaced. Get the timing checked. Drive for an hour at freeway speeds before getting it smogged. Let the engine idle if you have to wait at the smog station.

Your car is old. If it still does not pass. It is time to sell the car and let the new owner fix it. Car might need a new engine.

There are 4 "carbon" concentrations on a smog report: high and low speed levels of carbon monoxide and of carbon dioxide. Carbon dioxide can't be too high; low carbon dioxide (CO2) means an exhaust leak large enough to let the exhaust be diluted with air; that is a job for a muffler shop. Carbon monoxide (CO) is produced when the fuel burns with too little air, it is a "mixture" problem that will need a mechanic to correct.

In general, emissions problems are not DIY affairs even for moderately experienced amateurs. It is hard to do anything meaningful without special and expensive equipment. I do my own just because I'm stubborn and have made a good living troubleshooting things that I have to fix, whether we have the right equipment or not.

A tune up and a new cat converter should fix it

A 1990 Civic will probably need a new catalytic converter if it's never had one and new oxygen sensor couldn't hurt either. With these two items you'd likely pass.