Do you live in a state with smog requirements? If your check engine light is on, you will not be able to pass smog. Generally you can go to any auto parts store and they can read the code for free, which you can then google to find out what your car's computer is trying to tell you.
The number 1 thing that causes the check engine light to come on is the gas cap not being on tight. If your check engine light comes on and you don't have other major symptoms, it would be a good idea to check to see if your gas cap is tight. If that was in fact the issue, the car will correct itself once it reads that sensor again and the light will go away.
The second most important thing to know is that if you are having difficulty passing smog (required every 2 years in CA) you can still pay your registration on time. Registration late fees go up exponentially and also very quickly, (like every 10-20 days) so if you pay your registration on time, you won't incur these fees. You will still be driving with an expired registration sticker, until you prove the smog is complete, but at least it won't cost you more than necessary while you figure out how to get your car to pass smog.
Reach out if you have questions about a quote you may have received from your mechanic to get your vehicle to pass smog!