Support is a price where buyers have repeatedly stepped in; resistance is where sellers have. They aren't exact lines — think of them as zones where the balance of pressure has shifted before and may again.
You find them by looking left. Where has price bounced or stalled multiple times? Horizontal levels that line up with several past reactions are the ones worth marking. The more touches, the more traders are watching it.
Levels are most useful for planning, not prediction. A break above resistance can turn it into new support; a failed break often snaps back hard. Use them to define where your idea is wrong, then size your risk accordingly.