I just flew into class B airspace without a clearance, what should I do now?
While still in the airspace, you should contact the controller if you can, since it may be important to safety.

After landing, you may get the dreaded phone number from a controller (probably tower or ground) which you're supposed to call and speak with someone from the FAA. You should not volunteer any information about the incident during this call without talking to a lawyer or AOPA legal services first.

Then, most importantly, fill out a report in the Aviation Safety Reporting System. Any information you submit will not be used against you in an enforcement action by the FAA, and in many cases, people who volunteer information through the system receive much softer treatment, and sometimes forgo enforcement all together.

The reason the FAA incentivizes this self-reporting is so they can better collect information about accidents which inevitably happen. If they know why you accidentally strayed into the bravo, maybe they can improve procedures or charts to help keep other pilots from making the same mistakes.