Do I need to be worried about taxes/business license on my side projects?

So I have my regular job, so taxes for that are pretty straight forward—just once a year with my w-2 info, etc. But I also have a side job that I am actually earning quite a bit of money with, in fact, more than my regular job. There are no other employees in this side job, it’s just me (making mobile apps). So, my question is, now that I’m making a lot of money with this side job, do I need to register for a business license? Do I need to pay quarterly estimated taxes to IRS or anything like that?

Thanks.