It can very much depend on the herbs you are using, and the methods by with you are making the potion/tea.
Are you brewing it, is it a diffusion, a decotion or a tincure, have you got equipment like a pestel and mortor which can also affect the quality of you substance.
You can't combine some herbs with others as they all have different effects on the body, like Issy said do your research first and thoroughly examine your methods before going ahead with the practical work.
Another good way to stay safe is the poison test, when you have made your potion, use only a tiny amount by rubbing it against your skin, this is the first test. If after half an hour your ok, ingest a small ammount.
These are all methods herbal practitioners use, so in a sense the same principles still apply with potions too. Hope all goes well :)