It takes some trial and error and time to develop kinesthetic awareness of your muscles. When doing chest exercises, retract your shoulder blades back and down, keep your chin tucked down as well, and try and squeeze your chest muscles on each rep of whatever chest exercise you are doing while keeping your core muscles tight for the entire set.
Wearing compression clothing helps you to be more kinesthetically aware of your muscles as well, and gives your muscles something to press up against.