Hello MoniLink,
Maybe you don't have to pay any tax for you have only a small trade (in German: Kleingewerbe). See ****://bit.ly/2c13Vc4, **** for "Umsatzsteuerberfreiung für Kleinunternehmer).
Detailed information about Kleingewerbe/Kleinunternehmer and taxes you can find here: ****://bit.ly/2c14ORN (German text, but very likely no problem for you ;-) ).
As far as I know you get your tax number automatically after you finished your registration Details: ****://bit.ly/2c16BWW
The service center of your local finance office or your local IHK (Industrie- und Handelskammer) can give you further informations.
You can find a tax accountant (Steuerberater) here: ****://bit.ly/2c181Rs (**** for term 'Steuerberatersuche) or here: ****://bit.ly/2c18JOn
Kind regards, Jan