Beginner Ts plant the bomb on the open default and immediately push for kills. Pro Ts plant where their teammates can trade them.
Default plant is the loudest spot — every CT angle covers it. Sub-defaults (under truck on Mirage, behind van on B Mirage) put you in a place where one teammate can trade your death.
When you plant, your screen is on the bomb — you can't shoot. If a CT walks the corner during plant, you die. The fix: have a teammate covering the angle, plant where they can see you, and they get the trade.
Some sites have "stage plant" spots — Inferno B-Coffins, Dust 2 A-Goose corner — where you can plant out of multiple sightlines. Learn one stage plant per site you take.