However means "even so" or, "on the other hand," and you use it to introduce a complication or contradiction. You might tell your friend, "I love your cooking. However, I already have plans for dinner tonight."
The adverb however has a few different meanings. It often shows contrast. You might be training to become a lawyer, however your secret dream is to dance. However also means "in whatever way." You might say to your father, "However you have to do it, convince Mom to buy me that car!" It also means "to whatever degree." However certain you are in your gut that Mr. Plum is the murderer, you need to wait for evidence before accusing him.