Hey welcome to the world of the employed! First you need to determine if your student B permit allows you to work and how many hours/week. This should be printed on the permit. For the next point, you are taxed at source on a B permit meaning that the employer automatically substracts tax from your salary every month and you receive directly your net salary. So you don't need to worry about that.

Regarding the mandatory health insurance I am not sure what applies for you, but I can tell you that if you do need to subscribe to an insurance, the canton authorities will send you a letter urging you to do so, otherwise they will choose one for you. So unless you receive the letter you can assume you are fine. Good luck!

I kept my UK health insurance going for at least a year before I receieved the letter advising me that I must change.  I changed and there was no penalty

and that was with a standrad working B permit

I finally found out today that I can keep my *free* insurance as long as I'm a student and working part-time!

Thank you all for your answers!

When I was on a B student permit and found work, I was told I then had to go to the Office cantonal de la population, complete a form, pay a fee (90chf I think for each employer) and they would approve it straight away. But I think you have to take a letter from your school saying that your grades are okay, and working and studying wont affect your studies.

The Geneva OCP will give you all the information over the phone.