Answer:
It depends on what you're trying to say, and how you would say it if you were by yourself. Some examples:
If someone asked you, "Who's going to the store?"
You would respond, "Tim and I are going to the store."
However, if someone asked you, "Is it OK if I borrow this movie?"
You would then reply, "It's OK with me and Tim."
The difference is how you would say it if Tim wasn't around - would "I" go to the store, or would "me"? Does this phone belong to "I" or "me"? Whichever way makes sense when it's just you, also makes sense when you're with someone else. Lastly, Tim should come before "I" ("Tim and I") but after "me" ("me and Tim").