What is a "material topic", and how do I decide mine?
A material topic is a subject where your business has a significant impact on people or the environment — significant enough that leaving it out would mislead a reader. Under GRI 3 you determine your own material topics through a documented process, and that process, not just the conclusion, is part of what you report.
It is not a list you are handed. A coffee shop, a logistics firm and a software company genuinely do have different material topics, and a report that treats all three the same is a bad report.
A workable process for a small business:
List what your business actually does, step by step, including who you buy from.
For each step, ask what it does to people and to the environment.
Rank those impacts by how severe and how likely they are.
Write down who you asked and how you decided — this is GRI 3-1, and it is required.