Thank you for your question. You can find the official information in English here: http://www.existenzgruender.de/EN/Home/inhalt.html
As you will see, there are different rules for each activity. You will have to check whether there is a Craft or Guild for each activity, and whether the equivalence of your educational or qualification certificates is required (for example, for translators). You do not necessarily have to start a company. You can be self-employed or freelancer (there is a difference: A 'freelancer' is one type of a self-employed person in Germany. Certain types of self-employed work, such as being a teacher, consultant, IT-programmer or lawyer are categorised as being 'freelance' (freiberuflich) in Germany. Becoming a 'freelancer' usually encompasses some 'brainwork' and you will have earned an academic degree for the work you do. However, you categorise as 'self-employed' (Gewerbetreibender) if you work as a self-employed estate agent, commercial intermediary, have a cafe. If unsure, you must ask the tax office for your personal circumstances.).
For self-employed, you will need to register at the trade office. For freelance, you should follow the rules explained here: http://www.expatica.com/de/finance/Tax-and-freelancers-in-Germany_100950.html (it is a bit more simple, but not by much).
I believe freelance work is what you are looking for, with a simpler registration and tax return. But you must alwasy check twice with the local tax office that none of what you do falls into the other, more restrictive categories.
I hope my answer was useful and look forward to your rating, which is essential to my activity.
Dr I L Vlad