It somewhat depends on what kind of homeopath you are seeing....but here are some generals things that tend to help everyone:
I would begin by writing down a brief timeline....what happened in your life, when and how it affected you. Again, brief-just jot down notes.
Then list all the things that affect you on a regular basis and take a significant amount of your time, energy and attention. What are the pressing issues?
Consider when else in your life these complaints have manifested. "The only other time I've had this was during finals in college, or when we moved to a new state when I was a kid, or after my first pet died I felt something similar to this."
Then think a bit about how those things present. By this I mean, "I get massive migraines, but they always hit when I am angry, they always start on the left side, they happen only in the mornings."
Talk about the quality of pain/discomfort if there is any. "I get massive migraines and it's like someone is sticking nails into my skull, or beating my head with a hammer, or a drill boring into my head" etc.
Then write down what makes each of those things worse, or better. For instance, "I get massive migraines, but I feel so much better if I can eat a hot meal, or if I can lie down" etc.
That should pretty much cover you! That will give you a big old head start. Good luck and have fun! Remember too that a homeopath's job is to tease out what they need to know.
