How do I tell manager/s that I am quitting?

Talk to your direct supervisor and let them know that you appreciate the work but you have endeavors to start off on your own and give him/her the written notice at that time.

On the notice, acknowledge your supervisors and compliment them on their help and encouragement, but that you want to start your own business and you need to move forward. Thank them for their patience and understanding... blah blah blah...

On another note, are you independently wealthy? Are you considering health insurance and retirement pay? Just be careful. Starting your own business has many setbacks. Many young people don't consider work benefits when making decisions like this. "I'm not sick, I don't need health insurance".... You may want to reconsider. Health insurance, paid sick leave, retirement, workers comp, vacation pay, life insurance,... not to mention... if you have your own company you can't collect social security either. If the company you work for now doesn't have these benefits, perhaps you should continue job searching until you find one that does... js.

Yes, you should definitely submit a written resignation notice. If you have a personal relationship with the General Manager and think that he/she may be affected by your leaving, I would say, yes, let him/her know as well.

Since you're a permanent employee you should write them a resignation letter and give them 1-2 weeks notice.

It must be in writing. In it, you should thank them for hiring you, mention what a rewarding experience it has been, and that you are regretfully leaving to pursue another opportunity. You should give this to your senior manager, and verbally tell your direct manager.

Yes, you should give them notice in writing. Also, if you were hired through an agency, you should send them a copy of your notice because your employer will want to be refunded for the fee they paid. Usually, they are only required to pay if you stay for a full year.

Have a chat with your direct manager and tell him you are handing in your two week notice. That's it.