What is the best way to prevent infection/pain on a tooth that needs a root canal ? my appointment is approximately in a month.

Not simple: Is the nerve in the tooth alive or dead? If dead, you may need antibiotics to hold you over. If alive, no antibiotics needed. OTC NSAIDs should help with pain/inflammation. Not much more to add without additional information.

Immediate: The best thing is to have treatment done as expeditiously as possible. If your Dentist can't move you4 appointment up, consider seeing a root canal expert, a specialist Endodontist.