Teens lie for many different reasons, which can take you off the path of whatever the lie was about in the first place. So, your first best step is to take care of the problem at hand, then deal with the issue of your teen lying.
Don't freak out, talk to your teen and find out why they lied to you. Is there an issue that that you both can work out so as to prevent further lying? For instance, is your teen avoiding doing their homework because they want to go out with their friends after school and they aren't finding the time to get it all done? If so, handle that. Then give your teen a consequence for lying, nothing huge. Talk about trust and how much it means to you in your relationship with your teen. Give them a list of things that they benefit from because of that trust. Things like being able to go out with their friends or having a part time job. Express your hope that your teen will not find the need to lie to you again and that your always open to talk to them about anything.
Do not fall into the 'but what if my teen is not lying' trap. If you feel your teen is lying that is enough to follow through with your parenting responsibility of discipline. Get past the guilt and realize that if you make a mistake here, it's okay. You are human! Parents make mistakes. You can fall back and regroup. But if you continue to allow your teen to lie because you do not have large neon signs pointing you to undeniable proof, your teen will learn that lying works, which is a character flaw that will follow them throughout their life. This is much harder to fix.
Here is an example of how you can handle your teen lying to you: Say your teen's teacher called and told you that there is a missing assignment that your teen needs to get turned in so that he can receive a grade this marking period. You ask your teen and he says he turned it in. You ask when and he gets defensive. You let your teen know that your teacher called and the teacher does not have the assignment. You tell your teen to take out what they need to get the assignment done, they can have their privileges back when the assignment is complete and you see it. Then offer to help with completing the assignment.
After the homework is complete, talk to your teen about the lying. If this is the first time, you can let it go at the talk. If this is a continual problem, assign a consequence that makes lying not their best option in the future, but is quick so that you will be able to use it again should you need to.
Now if your teen insists they turned in the homework, call the teacher back and ask them if it is a possibility. I have had it happen that the teacher rechecked their stack of papers and lo and behold there was the assignment from my teen. Teachers are human and make mistakes too. But if the teacher does not have it you teen will need to complete it, even if they did do it the first time.
What if I don't know if my teen is lying? Try this quiz to see if it can help: Quiz: Is Your Teen Lying?