Need help regarding project

My project is:
For Mean:
1. how can i fix a margin of error? can i suppose it?
2. do i collect sample first?
3. is there any source to get sample data?
Highly appreciate your efforts, Thank your


Less is more. Stay pure. Stay poor.
Whenever you conduct a study, you typically conduct a sample size calculation if there is an inference you are going to test. At this part you have to have a small sample, meta-data on the population, or make an educated assumption on values of variability. So you likely need to pick one of these options.