How to take a clear picture in the mirror?

It's unprofessional of you to shoot like this, and the effect will not be very good. First of all, you must have a tripod to have a relatively stable image under the slow shutter (of course, you can't move). If you have a remote control, you can use it to delay taking selfies. If not, you can postpone taking a selfie for a long time. The ideal situation is to use indoor lights.

It is not recommended to shoot in front of the mirror, it will not be very clear. You can't shoot directly unless your mirror is clean. If you have to shoot in front of a mirror, please use a tripod.