I'm a psychologist, PhD based in Germany, starting up a business online in English. For the moment it is only a Blog, but I aim at monetising it in time (not doing therapy online but coaching, mentoring and probably building up a closed forums.) How do I protect myself and my future customers? May I copy legal documents from other, similar websites (self-development, relationships...)?
You can easily find boilerplate legal wording out there. If you run a Google search for each key term such as "boilerplate terms of service" you'll find quite a few resources. If you want to copy and paste, make sure it is covered under the appropriate licensing that allows this, such as Creative Commons. You can read more on that here https://en.wikipedia.org/wiki/Creative_Commons_license
The extent of protection you provide to yourself and future customers will likely depend on the amount of time and money you invest in legal advice. Your best bet is to consult an attorney that specializes in copyright law so your exact platform and business model is adequately covered.
You can copy and paste boilerplate language but you could run the risk of opening yourself up to legal liabilities that are not covered. Do plenty of research!
*I am not an attorney and this answer should not be construed as legal advice.*
Google each, then cut + paste them onto your site.
Most of these are similar.
Make changes where required.