An embouchure is the way you position your mouth on the mouthpiece. For the clarinet the embouchure is much different than other instruments. For the clarinet you have to fold your bottom lip over your lower teeth and put your upper teeth on the top of the mouth piece. If you push too hard with the bottom lip on the reed you will get a squeak. You should never get tighter on the embouchure for different notes, always keep your embouchure calm. When you are beginning you should take your time with getting used to the embouchure. It might feel uncomfortable at first but you will eventually get used to it.
The breathing for the clarinet is also a big part. My teacher taught me the wolf breathing method. It is when you keep your mouth in a still position and take in a quick breath. It can help you when you need to take breaths quickly in fast pieces. Honestly, breathing can be the hardest part while playing the clarinet for me because you need quick breaths and they can run out quickly. You just got to keep on practicing your breathing to get better at the quicker breaths in.
Here is a website that might help you when playing the clarinet : The Clarinet Net