In case someone raises an opposition against your application, the IP office will notify you by sending a notice of opposition. All related documentation will also be provided so you or your attorney can examine it. You will be asked to supply a response within a given timeframe, the length of which depends on the jurisdiction.
It is advisable to hire an attorney who can act on your behalf, as opposition proceedings might be very complex. The reasons for filing the opposition might vary, so inspecting them thoroughly is important. Your attorney will then determine the best course of action.
You can read more about the topic in our article What happens if someone opposes my trademark application?