Start with the basics. The most basic is to simply sit quietly and pay attention to everything that is happening, thoughts, breath, heartbeat, emotions, sounds, sensations, etc. Just pay attention. This can be done before concentration practices like focusing on and counting the breaths. Just sit quietly and pay attention.
There is a practice called “Noting” that also helps with this. When a thought comes, say “thinking”. When you notice a sound, say, “hearing”. When you notice a feeling, say “feeling”. When you notice a sensation, say, “sensing”.