ここに　(koko ni) means "here" and indicates the location. Think of the particle に (ni) as a finger that points directly where you are standing when you think of ここに！It's often used when verbs of motion are used (come, go, etc.). For example: 明日はここに来ます。　(ashita wa koko ni kimasu) "I will come here tomorrow." ここで (koko de) means "here" in the context that a particular action is occurring in a specified location. Think of being in a particular place doing the same thing all "day" (sounds like "de," eh?). For example: ここで本を読みたいです. (koko de hon wo yomitai desu.) "I want to read a book here." I hope my examples and mnemonics help you. If you have more questions, let me know.

Kokoni is To here. Kokode is At here.