First, you have to create a tax account if you just arrived in France or you are attached to your parents’ tax declaration.
You can create an account by sending an email using this contact form. You will get a tax identification number to access your online account.
You can estimate your income tax using this online simulator.
You have to submit an annual income tax declaration form every year around May or June. The information about your salary is directly given to the tax authorities by your employer. You can also declare the donations and pensions you have given. If you pay more income tax than you were liable, you will get a tax refund. This refund will be sent to the bank account (RIB/IBAN) you have registered in your tax account.