There are small number of general legal requirements for setting up a business in Germany. For example, you must be of age, i. e. you have to be at least 18 years old. Another requirement is that you must have never been banned from practicing the profession you intend to set up a business in. Such a prohibition can be imposed by courts after serious criminal offences.
Before setting up your own business, you must register your planned activity. You can do this either at the trade office (Gewerbeamt) or the tax office (Finanzamt) – depending on whether you intend to work on a self-employed basis (gewerblich) or as a freelancer (freiberuflich). For some professions you must prove that you do not have a criminal record. For that purpose, EU citizens residing in Germany can usually apply for a "European Certificate of Good Conduct" at their local Residents' Registration Office. If you are from a on-EU country, you must obtain comparable proof from the competent authorities in your home country.
Additional information on the formalities you have to go through as you set up your business is available here.
Last but not least, you should ask yourself if you are ready to be your own superior. To assess your personal prerequisites, you can complete this online test (available in German only). If you have any doubts, it may also be worth seeking advice from a start-up expert.
Recognition in Germany