If you launch a business by yourself you are automatically a sole trader.
Small business enterprises do not need to be registered in the Commercial Register. They only need to be registered at the business registration office (Gewerbeanmeldestelle) of the local authorities. You have sole responsibility and are liable without limitation with your personal assets. §§ 1 – 104 of the German Commercial Code (§§ 1 – 104 Handelsgesetzbuch (HGB)) covers legislation on sole proprietorships.
You can also register yourself in the Commercial Registered as a "Registered Merchant". It could be useful for your public image. You are thus regarded as a general merchant and are required to carry out commercial bookkeeping and accounting.
Note: Commercial bookkeeping is mandatory if your turnover exceeds 500,000 euros per annum or you make more than 50,000 euros profit per financial year. Sole traders with a turnover of up to 500,000 euros and a profit of up to 50,000 euros per fiscal year are exempted from the obligation to keep accounts, inventory and draw up a balance sheet as specified in the provisions of commercial law.
If the trading activities are fully commercial, you as a sole trader will be required to register the business in the Commercial Register.
Tip: You can find detailed information on setting up a sole proprietorship (Informationen zur Gründung eines Einzelunternehmens) for example on the Business Start-up Portal of the Bundesministerium für Wirtschaft und Technologie (BMWi).
In cases of doubt or if you have any questions or problems, please contact the relevant authorities directly.