The fast rule
Do not board by destination alone. Multiple Shinkansen trains can go toward the same region. Your reliable identifiers are train name, train number, departure time, car number, and seat number.
Before the gate
Check whether your ticket is paper, QR, IC-linked, or a pass reservation. Some trips require both a base fare and a limited express / Shinkansen ticket. If the gate rejects you, step aside and ask staff instead of trying repeatedly.
Look for the car number markings on the platform floor or overhead signs. Reserved and non-reserved cars can be in different parts of the train. If you have a seat reservation, line up at that exact car. If you have a non-reserved ticket, line up for a non-reserved car and board early enough to find seats together.
Luggage decision
Large suitcases make boarding stressful. If you have oversized bags, check the operator rule before travel and reserve the correct space when required. For a multi-city trip, luggage delivery can be easier than carrying bags through busy transfers.
When you are unsure
Show staff your ticket screen and ask which platform and car position to use. The most useful phrase is not a long sentence; it is the train number plus Where should I stand?